The National Organization for Marriage (NOM) announced Thursday they will file suit against the IRS, accusing the agency of violating federal law by leaking the organization's private donor list in 2012.

NOM, an advocacy group opposing same-sex marriage, has claimed for months that its private donor list was illegally disclosed to the Huffington Post, and the Human Rights Campaign -- a top advocate for gay rights and gay marriage, whose former president served as a co-chairman of President Obama's re-election campaign.

Releasing confidential taxpayer information is a felony. Cleta Mitchell, an attorney for the Act Right Legal Foundation, which is handling the case, has pressed the IRS and the Treasury’s inspector general for tax administration to find out who was responsible.

NOM President Brian Brown accuses the IRS of stonewalling and says the group has proof that its tax return was leaked.

“This is a federal crime,” Brown said. “Worse, the confidential information contained in the illegally leaked documents included the identity of dozens of major donors and the HRC used the confidential donor information to harass our donors. This is a chilling set of circumstances that should ring alarm bells across the nation.”

The IRS, which has had most of its employees furloughed in the government shutdown, and HRC did not immediately return a request for comment.

NOM complained about the disclosure of its donor list when it first occurred in February 2012 but the controversy garnered more attention after disclosures that the IRS targeted conservative nonprofit groups that had applied for tax-exempt status.

Congress and the IRS inspector general are still investigating the matter.

During last year's presidential race, HRC posted the donor list online.

The same day a report in the Huffington Post said it had received the list separately from HRC and highlighted a $10,000 donation from the political action committee affiliated with 2012 GOP presidential nominee Mitt Romney.

Both HRC and Huffington Post described the person who leaked the donor list as a “whistleblower.”

Founded in 2007, NOM was created to fight state proposals legalizing same-sex marriage, particularly Proposition 8 in California, and advocate on behalf of traditional marriage. It has a nonprofit advocacy arm, which raises money for its efforts.

John Eastman, NOM's chairman, says the timing of the leak is particularly suspicious. The day after HRC posted the documents, their president, Joe Solmonese, left to become a co-chairman of Obama's re-election bid.

After NOM complained, HRC took the information off its website, but a link to the Huffington Post article is still active.

There are many emerging examples of how the IRS has targeted conservative groups, Eastman said, but this incident, handing over private donor lists of one group to its political enemies, is “a special category of abuse and concern.”

“Our lawsuit will be a powerful tool in ending the stonewalling and getting the truth to the American people,” he said.

He urged Congress to pressure the IRS, Treasury Department and other officials to help identify who is responsible for releasing the confidential donor information.