WASHINGTON - The Federal Election Commission is questioning the Nassau County Republican Committee about its $50,000 contribution to a Massachusetts-based super PAC that ran ads against Democratic House candidate Kathleen Rice just days before the Nov. 4 election.

Exactly how the donation came about remains a mystery.

The FEC on March 2 asked for more information on how the money was raised and spent. It sent the requests after the Nassau Republicans registered as a federal party committee and filed a campaign finance report on Jan. 31 -- long past the agency's deadlines. The late finance report could lead to an FEC fine.

The reports and the FEC inquiries disclosed for the first time who was behind the 11th-hour Internet ad buy attacking Rice by the obscure super PAC, called the Independent Majority Group. Rice, then Nassau's district attorney, defeated Republican Bruce Blakeman with 53 percent of the vote.

Neither Nassau GOP officials nor the super PAC's administrator and lawyer answered questions from Newsday about why a Long Island political party would give to an out-of-state PAC, why the political action committee was created and why it focused exclusively on the Nassau race. Blakeman also did not respond.

"This is an excellent example of how the FEC reporting system can be gamed to make sure the true source of the funding for last-minute independent expenditures isn't publicly disclosed until after the election is over," said Washington election lawyer Brett Kappel, a partner at Akerman LLP.

Kappel added that the Nassau Republicans also could face scrutiny to determine whether there was any illegal coordination, which prohibits candidates or the candidate's party from cooperating or consulting with a super PAC on the content, placement or timing of the PAC's political ads.

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Rice spokesman Eric Phillips said, "We're certainly exploring our options" about filing a complaint.

Nassau Republicans' counsel John Ryan and spokesman Anthony Santino said in interviews that the committee made errors in filing its paperwork.

In answer to one FEC query, Ryan said the Nassau committee is separate from the New York State GOP. In the other, Ryan said the finance reporting mistakes will be fixed by an April 6 FEC deadline.

But they said the committee had not coordinated with the super PAC on the ads.

"I couldn't tell you how we found the super PAC or how the contribution was caused to be made," Santino said. "But in the closing days of a campaign, we make a lot of contributions in a lot of places."

A legal adviser who worked for both the super PAC and Blakeman -- Vincent DeVito, a partner at Bowditch & Dewey in Boston -- also said the arrangement did not violate the bar on coordination. DeVito said dual roles such as his do not suggest otherwise.

"There is nothing weird or unique about that," he said. "The use of common vendors is allowed." He said client confidentiality barred him from discussing further details.

The Independent Majority Group was registered by Nick Connors of Lynnfield, Massachusetts, as a super PAC in July 2014. Super PACs can accept donations of unlimited amounts for independent expenditures for or against candidates.

On Oct. 29, six days before the election, the Nassau Republicans contributed $50,000 to the super PAC and a longtime Nassau GOP donor, Long Island real estate developer David S. Mack, gave it $10,000, the PAC's FEC filing said. Mack did not respond to a query.

The super PAC spent $45,000 on ads on Oct. 31 and $5,000 on ads on Nov. 3. The remaining money paid fees for Connors, who was the administrator, and DeVito.

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Among the 102 single-candidate super PACs in the last election, the Independent Majority Group was unusual -- it was one of only two funded by local parties, said the Center for Responsive Politics, a nonpartisan group that tracks political money. Interest groups and individuals funded the others.

The Nassau GOP was required to file as a federal party committee 10 days after it made the contribution because it exceeded $1,000, said Kappel and Jan Baran, an election law specialist and partner at Wiley Rein in Washington. But it didn't file until nearly two months later.

Federal committees must submit regular campaign finance reports, including one after the election and another for the remainder of the year, they said. Nassau Republicans only filed the year-end report.

The county party could be penalized under the FEC's automatic fine system for late reports or through further investigation by FEC staff, Baran and Kappel said.