Outside special interest groups spent $3.8 million on behalf of the two finalists in Boston's mayoral election, pumping an unprecedented total of independent expenditures into a race that shattered local campaign spending records.
The groups spent twice as much on behalf of Mayor-elect Martin J. Walsh as they did on John R. Connolly.
Labor-affiliated political action committees lavished nearly $2.5 million on Walsh's campaign, while local and national education groups spent roughly half that amount on Connolly's bid, according to a report Tuesday by the state's Office of Campaign and Political Finance.
The campaigns themselves spent a record-setting $5.6 million from their own accounts combined, shattering the campaign spending record set in 2009 when current Mayor Thomas M. Menino and challenger Michael Flaherty spent a combined $4 million, according to the report.
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In all, a combined $9.4 million was spent by the two campaigns and by the outside groups on the candidates' behalf. But it was the heavy involvement of outside independent expenditure groups, unleashed by a 2010 Supreme Court decision, that alarmed government watchdogs and political observers.
"The amount of outside spending in the Boston mayoral race was remarkable," said Pamela Wilmot, executive director of Common Cause, a nonpartisan advocacy organization that has called for tighter controls on outside spending in elections. "It shattered the ceiling in terms of what we've seen before in a municipal race."
Special interest groups such as labor unions and political action committees have two ways of spending money in elections. They can give limited donations directly to a campaign, and they can also make much larger "independent expenditures," money spent on behalf of the candidate but not handled by the campaign. Independent expenditure groups are legally barred from coordinating with a candidate's campaign.
Tuesday's report sheds light on overall spending totals, but does not specify where the money spent by outside groups came from. State law allows their donors to remain confidential until January.
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The state's top campaign finance officials, news outlets, and the candidates themselves asked the groups to voluntarily release names before the election, but they declined.
Early in the general election campaign, Connolly said he and Walsh should swear off outside money, but gave up after Walsh refused and began allowing outside money to be spent on his behalf.
"We knew it was going to be a game-changer," said the Rev. Miniard Culpepper,who campaigned for Connolly. "One million extra dollars [for Walsh] in a race won by four thousand votes — that says it all."
Much of the outside spending came from Working America, the political arm of the AFL-CIO, which spent on behalf of Walsh, and Democrats for Education Reform, a Washington, D.C.-based group that spent on behalf of Connolly.
During the campaign, Walsh conceded that outside groups were spending heavily, but he stressed that campaign finance laws prohibited him from coordinating with those groups.
"The mayor-elect appreciates the support he received from over 6,000 volunteers and over 3,000 individual donors who supported his vision for Boston," Kate Norton, Walsh's spokeswoman, said in a statement to the Globe Tuesday night.
Walsh was the beneficiary of hundreds of thousands of dollars from groups whose funding sources and stated goals remain cloudy. One group, American Working Families of Alexandria, Va., spent more than $1 million on Walsh's behalf during the course of the campaign.
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The only name associated with the group is Bud Jackson, a Democratic consultant who repeatedly declined to disclose donors before the election.
In early November, a newly formed political committee called One Boston spent $480,000 on a television advertising buy on behalf of Walsh.
One Boston listed no specific political goals and the only name linked to the group's paperwork is Jocelyn Hutt, a 55-year-old woman from Roslindale who city records show had not voted in three of Boston's past four municipal elections.
The secretive nature of a significant chunk of the money spent by outside groups in the race has led several state lawmakers to join with good government groups to explore proposals to tighten disclosure rules.
"The groups are finding lots of ways of evading even the disclosure rules that do exist," Wilmot said. "We need to tighten all of that up to give the public the most information possible before they're at the ballot box."
Wesley Lowery can be reached at wesley.lowery@globe.com. Follow him on Twitter @WesleyLowery.