In October, Pasceri’s mother had undergone surgery at the hospital, performed by Dr. Michael Davidson, a highly respected cardiovascular surgeon known for his skill in operating on the most fragile patients. But Marguerite Pasceri, 78, was very sick, and she died in November. While her death certificate lists her cause of death as “cardiovascular collapse” caused by her preexisting illnesses, her son had become fixated on the idea that she was killed by a post-operative drug Davidson had prescribed.

In the thumb drive files, Pasceri laid out his research on the drug, amiodarone, and said he was “certain” that Davidson was responsible for his mother’s death, according to a person who has examined the drive. “Not solely,” he wrote, but “the majority.”

Pasceri also referenced the Psalms and stated starkly:

“The doctor is dead. I am dead. There is nothing more anyone can do.”

In the days after Jan. 20, when Pasceri shot and killed Davidson, the portrait of Pasceri that emerged was of a man who simply snapped with grief about his mother’s death. But court records and interviews with Pasceri’s family members reveal he had been troubled for decades, with a deep well of anger and an unshakable belief in his own righteousness, even when acting at the expense of his closest relatives.

And a Globe review of evidence in the case, including hospital surveillance video; information provided by the lead investigator, Boston Police Sergeant Detective Richard Lewis; and interviews with hospital employees who interacted with him, provides the first detailed picture of Pasceri’s wanderings in the hospital that day — a random, unaccountable meander that stretched over an hour — and of his encounter with Davidson. Witnessed by a colleague, the doctor made a calm and patient effort, in the midst of a typically busy day, to understand Pasceri’s complaint.

And then three shots.

Calmness amid a busy hospital

Security cameras at the hospital captured Pasceri entering the hospital through the revolving door at 75 Francis St. at 8:48 a.m. and walking toward the restrooms in the main lobby.

In the blurred images, he appeared calm. Around him, the business of life and medicine hummed along: a person stood at the front desk, others walked or paused in small clusters. If they noticed Pasceri, nothing registered; there was no reason to pay heed to the heavyset man with graying hair in khakis and a black jacket, his face expressionless, passing among them anonymously.

At 8:54 a.m., a camera picked him up passing the gift shop, on his way to the glass bridge that crosses Francis Street, connecting the Brigham’s main campus to the Shapiro Building where Davidson was working in the Watkins Cardiovascular Clinic.

At 8:56 a.m., Pasceri made his first appearance outside the clinic. Then he disappeared from the frame.

Courtesy of Suffolk County District Attorney's Office

Stephen Pasceri captured on hospital surveillance video on the morning of January 20.

S

tephen Pasceri presented himself to the world as a devoted husband and father of four, a churchgoer who spent hours volunteering at the simple white Millbury Federated Church on Main Street, just around the corner from his home.

His pastor, the Rev. J. Clifford Davis, Jr., said Pasceri started a men’s group to do home repair projects for widows and single mothers, and he went on missions to rebuild churches damaged in acts of racial intolerance, crime, or natural disaster. At a weekly men’s prayer breakfast every Thursday, Davis said, Pasceri would pray for his family, and give thanks for his wife, his job, and his church.

But there was another side to the man, known best to those close to him. He could be self-righteous, judgmental, quick to anger, and occasionally violent, several family members said.

“Really deep down, inside, he must have known that he was wrong for some of the things he did,” said his sister, Marguerite Joly. “I think he slowly lost his mind.”

Pasceri’s brother, John Pasceri, said he didn’t think his brother was mentally ill but acknowledged that his hard nature could cause discord.

“When you were wrong, you had to suffer the judgments of Steve,” he said.

A young man turned away a reporter at Stephen Pasceri’s home, and his wife, Teresa Pasceri, declined through her pastor to be interviewed.

Stephen Pasceri grew up the oldest of four children in Worcester, surrounded by extended family. As a child, he could be indifferent to the feelings and safety of his siblings and his best friend, a cousin his age, said Joly and an aunt, Patricia Bastille. But no matter what he did, Bastille said, his mother could not bring herself to punish him.

“And it’s too bad because that was his downfall,” she said.

At 17, the family members said, Pasceri was arrested for stealing guns from a neighbor’s house and stashing them in his younger brother’s closet. Bastille said she and her mother hired a good lawyer, and he was not convicted. Instead, he got a deal that included counseling and service in the military. Court records from this period are not available.

He served three years in the Army and returned home in December 1980, seemingly a changed person, Joly said.

A photograph from those years shows her brother standing on the front porch of their childhood home on Schussler Road, shirtless in a summer thunderstorm. His back is toward the camera, his arms are raised toward the sky.

“He was so happy,” she said. “He was.”

Approaching the clinic again

One minute after first passing the clinic, Pasceri walked by again. He was captured on video returning to the area just outside the door. He didn’t go in.

Instead, in a nearby common area, he took off his jacket, rolled it up, and stashed it behind some chairs and potted plants. Then he headed back toward the bridge.

At 8:58 a.m., he appeared on camera again, headed toward Au Bon Pain, where he bought a bottle of water. Five minutes later, he returned to the Shapiro Building, picked up his jacket, and headed for the elevators.

There is a shot of Pasceri at 9:08 a.m. standing in an elevator with three other people. He stood with his back to the wall by the button panel, his jacket draped over his left forearm, the bottle of water in his hand.

He had clearly seen the camera: He stared directly at it. His expression was flat.

A

few years after he got out of the Army in December 1980, Pasceri married and had his first child. He was managing properties in Worcester for his aunt Patricia Bastille, who had moved with her husband to Florida.

Looking to give the new family a good start in life, Bastille said, she and her husband sold Pasceri their home on Hawthorne Street in Worcester at a discount and extended him a loan of $25,000.

But Pasceri was a man who “systematically removed family members from his life,” his sister said, and he was about to take aim at Bastille.

Bastille said he abruptly stopped returning her calls, and then, after a technicality involving the $25,000 loan complicated his plans to take out a second mortgage, he was furious, she said. From that moment, their relationship was over. He would eventually try to destroy his mother’s bond with Bastille, telling his mother he would keep her from his children if she saw her sister, Bastille said.

“He cut us off, that’s what he did,” Bastille said. “He didn’t need us anymore.”

Pasceri’s ire could be unpredictable and out of proportion.

One night when he and his brother John were eating supper at their parents’ home, Stephen became enraged at the size of John’s helping of roast beef, John Pasceri said. Both men were in their 30s or late 20s at the time, he said. As John walked away, he said, Stephen tackled him from behind. John still had his dinner knife in his hand, and when he punched Stephen in retaliation, he inadvertently stabbed him in the stomach.

John Pasceri spent a weekend in jail, he said; Stephen decided not to press charges. But, John Pasceri said, unlike other family members who ran afoul of Stephen, he and his brother worked it out, and became closer.

“If he was on your side, there was nobody better,” Pasceri said. “But if he was against you, you didn’t want to be against him.”

In 1994, Stephen Pasceri’s relationship with his sister, Joly, exploded when he threatened to sue her and her husband, an electrician, because of a spat about work her husband offered to do on Pasceri’s house that he later had to put off.

Joly said she begged him to reconsider, asking him to think of what it would do to their relationship and their children. But he wouldn’t listen.

“This has nothing to do with you,” he told her, Joly recalled. “It’s only business.”

It was typical Stephen, Joly said: mathematically certain that he was right, and convinced he was absolutely justified in exacting retribution.

There were, however, occasional upwellings of tenderness.

When Stephen Pasceri’s father died of a massive heart attack in 2011, Pasceri’s brother Gregory recalled a telling moment in the hospital room. Stephen, he said, leaned over their father’s body, crying, and brushed his hair back from his forehead.

“I’m so sorry for everything,” he murmured. Gregory Pasceri and Marguerite Joly took this as a rare moment of self-reflection.

I

n early October, Pasceri’s mother, Marguerite, arrived at the Brigham for surgery to repair a valve in her heart. A vibrant woman with a love of books, she had smoked most of her life and suffered from several health problems. She had been diagnosed with emphysema in the 1980s and also suffered from chronic obstructive pulmonary disease. She had already endured two heart surgeries.

But delicate hearts like hers were Davidson’s specialty. Fellow doctors recalled him as a brave surgeon willing to operate on the riskiest patients — with a remarkable level of success. He was beloved for his bedside manner, and for his willingness to talk patients’ families through the intricacies of a surgery — no matter how long it took. Marguerite Joly had complimented him on his “magical hands,” and he received a constant stream of such letters and gifts of thanks.

Marguerite Pasceri’s surgery went well, but she soon took a turn for the worse, and in late October, she was taken from a rehab facility to Saint Vincent Hospital in Worcester. There, her health steadily declined. Stephen Pasceri, the eldest child, was her health care proxy, and Davidson’s assistant said he called the surgeon’s office seeking advice. He asked whether Davidson could look over his mother’s medical records to ensure St. Vincent’s was treating her properly. Davidson was happy to oblige.

On Nov. 15, the family made the decision to remove her breathing tube, in accordance with her wishes, and allow her to die.

As the family gathered around in the hospital room and played her favorite piece of music, Handel’s “Messiah,” Joly leaned over her, pressing sponges soaked in cold water to her mouth.

“Mom, can you see Dad?” she asked her mother, recalling for her mother the summers she’d spent sitting with her love in the backyard. “He’s sitting there in the glider swing. You need to go over to him.”

In that final moment, Joly said, her mother smiled. Her family took comfort that her end was gentle.

But not her son Stephen.

As his mother drew her final, peaceful breaths, he turned and walked out of the room, pressed his face into a pillow, and screamed.

‘I’m not leaving without seeing him’

Just after 9 a.m., he got off the elevator on the Brigham’s first floor and a camera recorded him sitting for a moment then getting up and walking out of the hospital and back to the garage. For a few minutes, he was off camera, out of the hospital. By the time returned to the hospital at 9:19 a.m., he no longer had his jacket. And holstered on his right hip, tucked beneath his sweater, police said, was his gun.

Inside, he walked into the chapel, where there are no cameras. His time there was unrecorded and lasted less than two minutes.

At 9:22 a.m., Pasceri bought a newspaper in the main lobby.

At 9:24 a.m., he walked to the Shapiro common area near the clinic on the second floor and sat down. He looked at his phone but made no calls, sent no text messages.

At 9:41 a.m., the cameras captured one final image of Pasceri outside the Watkins Clinic.

This time, he walked inside and up to the reception desk.

“I want to see Dr. Davidson,” he said to the secretary, Cheryl Cantalupo. That wasn’t how it worked, she told him: He needed an appointment. His face was completely blank, she said; his voice emotionless.

“No, you don’t understand,” Pasceri said. “I’ve traveled a long way. I have to see Dr. Davidson. I’m not leaving without seeing him.”

He repeated it so many times, Cantalupo thought he must be hard of hearing.

F

rom inside the clinic, Pasceri called Davidson’s assistant, Mishel Burgos, and told her he was in Boston and wanted to see the doctor. “Of course,” Davidson told Burgos when she relayed the request.

At first, Davidson offered to meet with Pasceri in the afternoon, after his clinic, so he would have ample time. But Pasceri was adamant, and Davidson was accommodating. So Pasceri was led back to Exam Room 15.

Joly said the family had discussed talking to a lawyer about her mother’s death, and one of their concerns was the continued use of amiodarone, a drug that regulates heartbeat. But, she said, she had no idea her brother had become so obsessed with their mother’s death, or that he had amassed a huge amount of research about the drug and its side effects, which he left on the thumb drive with his undated final letter. John Pasceri declined to provide a copy of the thumb drive but said it contained a detailed timeline of his mother’s illness, her use of the drug, and his brother’s conversations with Davidson. A civil lawsuit would not be enough for him, Pasceri said in his letter. He said he was sorry to leave his brother with such a mess.

While Pasceri waited in the exam room, Davidson pulled up Marguerite Pasceri’s record on a computer and looked it over with a physician assistant, Sarah St. Jean, in another room.

Davidson, 44, had been at Brigham since 2006. He lived in Wellesley with three young children and his wife, plastic surgeon Dr. Terri Halperin, who was seven months pregnant with their fourth, a baby girl.

Everything was right in their world. Davidson loved to take his older children, Kate and Liv, 9 and 8 years old, frogging and fishing and backyard camping, and he was teaching them to ski.

He played guitar for his 2½-year-old son, Graham; he ricocheted between soccer games and school plays and groundbreaking surgeries. He delighted in taking photographs on vacations and then framing as gifts the ones his friends and staff liked best.

He was the kind of man who remarked so often, and so enthusiastically, on the beauty of the world around him that his daughters used to tease him about it, coining the term “Daddy-beautiful” for the moments that swept their father up in reverie.

As Davidson prepared to go talk to Pasceri on the morning of Jan. 20, according to police interviews, he remarked to another doctor, “Watch this, he’ll probably shoot me.”

He said it the way a person might say their wife or their father was going to “kill” them for some fleeting offense — it was a joke, hyperbole. On any other day, it would have been quickly forgotten.

A repetitive, but not heated, argument

Davidson walked into Exam Room 15 with St. Jean, the physician assistant, at 10:25 a.m. Pasceri seemed angry, St. Jean recalled, but he never raised his voice. He sat at a small desk kitty-corner to Davidson; St. Jean pulled up a rolling chair next to the doctor.

Pasceri’s gun was holstered underneath his pants, the grip sticking up over his waistband but covered by his sweater. If it bulged, no one noticed.

Right away, Pasceri told Davidson to open the Internet, go to Drugs.com, and look up amiodarone.

“Are you aware that this drug is extremely toxic?” Pasceri asked, St. Jean said, pointing to the website. “Do you see all of the warnings on Drugs.com?”

Davidson explained he was aware of all the side effects but said Marguerite Pasceri did not react badly and was being monitored. Any drug, he explained, even an antibiotic, has potentially dangerous side effects.

“Well, my mother died because of this,” Pasceri said, his face twisting into a snarl.

It was the only moment, St. Jean said, that his face showed any expression.

The conversation became repetitive, she said, and Pasceri did not seem to be listening to what the doctor was saying. As he had in the waiting room, Pasceri simply repeated himself again and again.

“You did not tell me or my family the detrimental side effects of this drug,” St. Jean recalled him saying. “And my mother’s dead because of this.”

Eventually, after about 10 or 15 minutes, Davidson turned to St. Jean and told her they were falling behind.

“Sarah, why don’t you go start and see the next patient?”

A last effort to warn others

Detectives will never know for sure what went on in Exam Room 15 after the door closed behind St. Jean. There are no cameras in the exam rooms.

What they do know is that Pasceri and Davidson remained in that room together for the next 25 minutes or so, and no one heard raised voices.

There was a father and son in the exam room abutting 15, and it was silent, they later told police, until 11:03 a.m., when they heard the sound of a table sliding across the floor. Then: three gunshots.

The first bullet hit Davidson in the left hip. The evidence suggests the doctor was standing, and that he spun around and reached for the door. The second bullet hit him in the back.

In the last conscious moments of his life, Davidson did not call for help.

Instead, witnesses said, he fled the exam room, and ran through the hallways screaming a warning:

“Run, run, he’s shooting, he’s shooting!”

Behind him, Pasceri put the gun in his mouth and pulled the trigger one last time.

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avidson’s wife is due to have their baby girl in April.

Terri Halperin’s family is struggling with the senseless circumstances of his death, she said in an e-mail.

“He can never be replaced,” she said. “My children asked why this man killed Daddy when he got to have his mother his whole life and they only had their Daddy for a short time. Sometimes the most brilliant questions come out of the mouths of babes. We are beyond devastated.”

Halperin’s attorney, Thomas Reilly, said the one small comfort that Halperin has taken from an event too terrible to understand is that her husband died as he lived: trying to save other people’s lives.

“Right to the end, he was just a wonderful human being,” Reilly said. “He’s a hero, and a hero who died in the line of duty.”

A fund has been set up to support the family, according to a statement from Brigham spokeswoman Erin McDonough.

“There are no words that can adequately describe the depth of the loss the Davidson family and the BWH community are experiencing as a result of a senseless, selfish act of violence,” McDonough said.

“We encourage anyone whose life has been touched by Dr. Davidson or who has been moved by his story to support the Davidson Family Fund” at www.rtn.org/davidsonfund.

Davidson’s assistant, Burgos, has left his office untouched: The bright hearts and snowy trees his children drew for him paper the walls, along with smiling family photographs. On his desk sit gifts from his patients, including a pink, flowered, handknit outfit for the daughter he never got to meet.

With Pasceri dead and the evidence clear, Boston police and the district attorney have closed the case.

“We keep an open mind and rule nothing out, but the only criminal conduct we’ve found was that of Mr. Pasceri, who is deceased,” said Jake Wark, spokesman for Suffolk District Attorney Daniel F. Conley, in a statement.

Stephen Pasceri’s family has been left to grapple with his final, terrible act.

“I wake up every morning, and I realize, ‘My god, my brother killed that man,’ ” Marguerite Joly said. “Every morning. And then I think about how they’re dealing with the day. How are they dealing with today?”

She does not know what could have prevented her brother’s last act — or if he was headed inexorably towards it all along. But she is focused, now, on what she can do.

She hopes to use some of the money from her mother’s estate to start a seed fund, which she ultimately hopes to use to create a center for grief and healing, for people who lose loved ones. It won’t bring Davidson back, but it is something she can do.

Another thing she can do, she says, is tell Stephen Pasceri’s story in the hopes that someone, somewhere, will read it and recognize in their brother or son or husband the warning signs she says her family missed.

“I think about all the people who are affected by his loss,” she said of Davidson. “He was an incredibly world-renowned skilled surgeon. My brother has caused a rippling effect on the world by killing this man.”

Evan Allen can be reached at evan.allen@globe.com. Follow her on Twitter @evanmallen.