The Iowa Ethics & Campaign Disclosure Board on Thursday voted unanimously to investigate the National Organization for Marriage (NOM).

The decision was in response to a complaint filed by Fred Karger of the gay rights group Rights Equal Rights on June 13.

Karger's complaint charges that NOM, the nation's most vociferous opponent of gay marriage, failed to disclose donors behind two campaigns aimed at ousting four Iowa Supreme Court justices who joined the court's unanimous 2009 ruling which struck down the state's laws limiting marriage to heterosexual couples. Three justices were removed from the bench in 2010, but an attempt to unseat a fourth justice in 2012 failed.

NOM has gone to great lengths in other states to avoid disclosing its donors. In May, Maine's highest court ruled that NOM must reveal the names of donors to its 2009 campaign to repeal a marriage equality law.

Marc Solomon, national campaign director of Freedom to Marry, responded in a statement: “For years, NOM has taken huge contributions from a few anti-gay donors in exchange for shielding their names from campaign finance authorities, whether or not that required flouting the law. Those days are quickly coming to an end. NOM's desperate appeals to the US Supreme Court have been rejected, and soon the public will see who bankrolls NOM's anti-gay campaigns, which – according to NOM internal documents – include a goal 'to drive a wedge between gays and blacks.'”

Karger has also filed a second complaint against NOM with the Federal Election Commission.