AUGUSTA — A national anti-gay marriage group that helped defeat Maine’s same-sex marriage law in 2009 may be fined more than $50,000 and ordered to reveal its donors, after an ethics investigation released Monday found that it violated the state’s campaign finance law.

Investigators with the Maine Ethics Commission said that the National Organization for Marriage’s failure to register as a ballot question committee and file campaign finance reports while playing a central role in the referendum to strike down the law was a “significant violation of the law.”

They also recommended that the commission order the group to file campaign finance reports with the state reflecting contributions and expenditures in support of the referendum.

“Maine people deserve to know who is funding political campaigns to influence their vote,” the report said. The commission will consider the investigators’ recommendations at a May 28 hearing.

NOM gave nearly $2 million to Stand for Marriage Maine, a political action committee, during the 2009 campaign. Gay marriage became legal in the state after a second referendum in 2012.

State law requires groups to register as ballot question committees if they raise or spend more than $5,000 to influence a state ballot question.

But NOM argues that none of the contributions were earmarked specifically to fight Maine’s gay marriage law and says it returned all contributions made by those who tried to designate them for the state’s referendum.

Brian Brown, president of the Washington, DC-based organization, said it plans to present its case to the commission later this month. He said the state is targeting NOM despite the fact that several other groups on the other side of the issue took the same actions.

“This is a witch hunt. It’s an attempt to go after a group like NOM because of its position on marriage,” he said. The group will appeal the ruling if the commission rules that they must pay the fine and file a report, he said.