When Mayors Against Illegal Guns Action Fund, New York Mayor Michael Bloomberg's vehicle for pushing for tighter gun control regulation, ran ads Sunday morning calling on President Barack Obama and his Republican opponent, former Massachusetts Gov. Mitt Romney, to offer plans to reduce gun violence, they became the first dark money group to publicly disclose its donors.

Bloomberg's group, a social welfare organization set up under section 501(c)4 of the tax code, is the first to comply with a U.S. District Court ruling requiring such organizations, which normally keep their donors secret, to disclose them if they run issue ads that mention federal candidates. The case, Van Hollen v. Federal Election Commission, was settled March 30, 2012.

The FEC announced that groups running "electioneering communications," or issue ads that mention federal candidates but do not explicitly call for their election or defeat, would have to disclose the names of all donors who contributed a total of $1,000 or more in 2012. Though the rule is retroactive--organizations that ran electioneering ads on March 30 onward are supposed to disclose donors--Mayors Against Illegal Guns Action Fund is the first to comply with it.

The ads aired in the wake of one mass shooting in Aurora, Colo. and, concidentally, on the same day as a second shooting at a Sikh temple in Oak Creek, Wis. Because they mentioned Romney within 30 days of the Republican National Convention, Mayors Against Illegal Guns had to disclose the spending on the ads. The Van Hollen decision, named after Rep. Chris Van Hollen, D-Md., forced the disclosure of donors as well. The bulk of their funding comes from Bloomberg himself, seen here greeting local Sikh leaders after the weekend shooting. The mayor, a publishing magnate, contributed $3.1 million of the $3.4 million the group reported raising. The only other sizeable contribution came from Los Angeles billionaire Eli Broad, who gave $250,000.

(Photo credit: Spencer T Tucker/Office of New York City mayor)