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A prominent anti-gay marriage group is accusing the Obama administration's IRS of leaking private tax files that listed Mitt Romney as a contributor -- documents which were later published by a group whose president is tied to the Obama campaign.

The National Organization for Marriage is on the warpath over the alleged breach. It has called for an IRS investigation and fired off a warning letter to the Human Rights Campaign, the pro-gay marriage group that first published the documents before taking them down.

The group claims it appears somebody within the IRS fed the Human Rights Campaign the documents listing 2008 contributors. On the list was a $10,000 donation from Romney's political action committee. In a blog in late March, the Human Rights Campaign declared it had uncovered one of the group's "top secret donors," and accused Romney of "essentially funding NOM's strategy of using racial division and unfounded scare tactics to attack LGBT equality."

The donation came as the National Organization for Marriage and other groups were fighting for the Proposition 8 measure banning gay marriage in California.

But the gay marriage debate aside, National Organization for Marriage President Brian Brown said the leak was illegal.

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"This is not something you'd expect to happen in the U.S.," he told FoxNews.com, noting that dozens of other donor names were on the leaked document. "The issue is this is a private tax document."

Brown has called for an investigation by the Inspector General for Tax Administration. His group, after analyzing the leaked documents, claimed the original versions have "official use" and other stamps on them indicating they were internal documents. Brown says that means somebody in the IRS leaked them, or somebody hacked into the IRS system to obtain them.

"This is a potential criminal investigation," he said, claiming the IRS has contacted the group and is "taking this seriously."

IRS spokesman Dean Patterson would not confirm whether an investigation is underway, but said the IRS protects taxpayer return confidentiality.

"IRS takes this confidentiality of return information very seriously. Any allegations of improper disclosures of taxpayer information are investigated by the Treasury Inspector General," Patterson said in a statement.

Brown's group was quick to point out that the Human Rights Campaign is led by Joe Solmonese, a national co-chairman of Obama's re-election campaign.

But the Human Rights Campaign adamantly rejected Brown's accusations. The two groups, to put it mildly, do not get along -- and the Human Rights Campaign cast the accusations as an attempt at intimidation.

"NOM's charges of illegal conduct by HRC are absolutely false," the group said in a statement.

"In the past few weeks HRC lawfully obtained and disseminated truthful information about NOM's racial-wedge strategy and secret donors. Noticeably absent from NOM's allegations is any awareness of First Amendment freedoms. Embarrassed that its true agenda is out in the open, NOM has launched a crusade to intimidate and suppress those who are revealing its anti-LGBT mission. HRC has no intention of helping NOM to suppress the truth."

The information on the Romney donation was actually available elsewhere -- including in a filing for the Romney PAC's Alabama chapter -- before the Human Rights Campaign posting.
Brown said his group was never trying to "hide" the information about Romney. But he said the tax form, of donations of at least $5,000, is supposed to be private and said the other donors on the list could be subject to intimidation.

The Huffington Post has also posted the tax documents, though the National Organization for Marriage has urged the website to take them down.