The National Organization for Marriage is filing a lawsuit against the Internal Revenue Service, after its confidential tax return was leaked from the agency to the group's chief political opposition.

"In March 2012 Human Rights Campaign posted a copy of our confidential tax return on its website...and we know for a fact that the source for this was within the IRS," National Organization for Marriage chairman John Eastman told CNA May 17.

The National Organization for Marriage qualifies as a 501(c)(4) nonprofit organization. They are obliged to make public their tax returns, " but there are parts of those tax returns that are explicitly confidential, including schedule B, our list of donors and their addresses."

On or about March 30, 2012, the Human Rights Campaign posted National Organization for Marriage's 2008 Schedule B on its website as a PDF.

"It had some redactions on it, and our computer guys were able to unlayer the PDF to get beyond the redactions and look at the original document, which is stamped with internal IRS markings," Eastman said.

Human Rights Campaign posted a version with retractions, showing a white bar diagonally across the pages. When this layer is removed, it reveals an Internal Revenue Service tracking number, as well as statements at the bottom and top of each page reading, "This is a copy of a live return from SMIPS. Official use only."

SMIPS is the tax agency's internal computer system. It is a felony offense for Internal Revenue Service officials to disclose private tax returns.

Eastman offered the three possible ways that the Human Rights Campaign obtained National Organization for Marriage's tax return from the tax agency.

"Either someone hacked into the IRS computer system...or someone fraudulently impersonated an officer of the NOM...or someone at the IRS disclosed this."

"Of those three, the one that's clearly the most plausible is the latter," Eastman said.

The tax return was quickly republished by the Huffington Post and other media outlets and blogs.

On April 11, the National Organization for Marriage requested that both the Treasury department's inspector general and the Department of Justice investigate the leak of their private documents.

When both departments, and the Internal Revenue Service, proved to be uncooperative, the nonprofit began filing requests under the Freedom of Information Act.

"Frankly we've been stonewalled...and they didn't give us any of the actually relevant information," said Eastman.

Having exhausted their means of recourse regarding investigations and requests for information, the National Organization for Marriage decided on May 6 to begin pursuing a civil suit against the Internal Revenue Service.

"Our final round of having to go through hurdles of FOIA requests is now concluded, and we're teeing up that lawsuit now," Eastman said.

Human Rights Campaign is an LGBT advocacy group promoting same-sex marriage. The group "had been trying to get our donor list for a long time, because then they can publish it on the internet and then people start harassing our donors and boycotting their businesses."

Eastman finds its significant that Human Rights Campaign's president in March 2012 was Joe Solmonese, who the month before had been named a co-chair of the campaign to re-elect President Obama.

"What a coincidence," Eastman said, that shortly after Solmonese was given a prominent position in Obama's re-election campaign, "somebody at the IRS discloses to that very same person our confidential tax returns, and commits a felony in doing so."

Eastman considers the idea risible that a low-level employee at the Internal Revenue Service would have taken the risk of committing such a felony without direction from a highly-placed supervisor.

"Given who was involved in this that we know, it seems pretty implausible," Eastman stated, that there wasn't "some involvement" from "high level political appointees at the Department of Justice or the Treasury department, as well as with the campaign folks."

The news of the pro-marriage organization's lawsuit comes as the tax agency is embroiled in scandal. On May 10, the government agency apologized for subjecting politically conservative "tea party" groups to additional scrutiny beginning in 2010. The agency asked some groups for donor lists, violating its own policies.

Since then, several nonprofit pro-life groups have also come forward with allegations of harassment and intimidation at the hands of Internal Revenue Service employees.

On May 15, Internal Revenue Service commissioner Steve Miller submitted his resignation to Treasury Secretary Jack Lew, who had requested it, becoming the first official to resign over "IRS-gate."

Joseph Grant, the commissioner for the tax agency's tax-exempt division, announced his resignation May 16.

At a Congressional hearing May 17, Miller told representatives of the House ways and means committee that the additional scrutiny given to politically and socially conservative groups was neither partisan nor politically motivated.