The chairman of an anti-gay marriage group testified Tuesday that his organization has proof that the IRS leaked confidential donor details last year, calling for prosecution into what he described as a "felony."

"This just smells and I hope this committee gets to the bottom of it," John Eastman, chairman of the National Organization for Marriage, said at a hearing before the House Ways and Means Committee.

Eastman testified Tuesday alongside several Tea Party activists who all claim they were targeted by the IRS. The Tea Party groups offered a first-hand account of how the IRS singled them out when they applied for tax-exempt status, asking them onerous questions and dragging out their application process.

But Eastman shed light on another potential controversy involving the IRS -- the unauthorized disclosure of tax document information. He recalled how information on their donors was leaked last year and published on the website of the Human Rights Campaign, which Eastman described as their "principal political opponent" on the marriage issue. The documents showed Mitt Romney's political committee as a donor.

Asked by Rep. Paul Ryan, R-Wis., if he had "proof" that the IRS leaked that material, Eastman said that he did.

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Eastman explained that while some information was redacted in the posted version, his group's "forensic" specialists were able to strip layers from the document and found "the original document that was posted there had originated from within the IRS."

He said the version had "internal IRS stamps," which "only exist within the IRS."

Eastman added: "You can imagine our shock and disgust over this. ... We jealously guard our donors."

He later alleged the information was "deliberately" provided to their opponents.

"If that's inadvertent, the word no longer means anything," he said, claiming his group has been "stonewalled" in its request for an investigation.

After a series of hearings on Capitol Hill where current and former IRS officials testified on the agency's actions, this is the first to feature alleged victims.

Karen Kinney, with the San Fernando Valley Patriots, described how she got a form from the IRS with 35 items divided into 80 "sub-points of inquiry," and was given just 20 days to comply.

The leader of a small South Carolina Tea Party group said her organization first applied for tax-exempt status in 2010 -- and is still waiting for the application to be processed.

Dianne Belsom, president of the Laurens County Tea Party, said her group in rural South Carolina has about 60 members.

For more than 18 months during the 2010 and 2012 election campaigns, IRS agents in a Cincinnati office singled out Tea Party and other conservative groups for additional scrutiny when they sought tax-exempt status, according to a report by J. Russell George, the Treasury Department inspector general for tax administration.

The report said Tea Party groups were asked inappropriate questions about their donors, their political affiliations and their positions on political issues. The additional scrutiny delayed applications for an average of nearly two years, making it difficult for many of the groups to raise money.

The Associated Press contributed to this report.