Tea Party leaders from around the country gathered in Washington Thursday to vent outrage at the Internal Revenue Service for targeting them and other conservative organizations seeking tax-exempt status and threatened to sue the agency for damages.

"They lost membership and donations, they missed the election cycle, they missed key moments, they were not able to bring speakers in," Jordan Sekulow, a lawyer representing Tea Party organizations, told reporters outside the Capitol.

The IRS last week confirmed that it had targeted conservative groups and the Justice Department responded Wednesday by announcing an investigation of the tax agency's practices. The IRS has since 2010 flagged for extra scrutiny groups that had "Tea Party," "Patriot" or 9/12" in their names. Some groups were kept waiting years for approval of their tax-exempt status, and a few had to wait so long that they simply disbanded.

The groups can sue for civil damages for the time and resources they spent dealing with the IRS, Attorney General Eric Holder told a House committee Wednesday, and Tea Party members said they're considering it.

Todd Cefaratti, founder of TheTeaParty.net, said he applied to the IRS for tax-exempt status more than three years ago but has yet to get it. The group had to respond to three rounds of detailed, intrusive questions from the IRS, he said.

"They wanted things like our donor lists, our private emails for the last three years, our Facebook communications and our Twitter communications," Cefaratti said. "We have over two million people on Facebook. This is private information. It has to make a chill go down your spine. Why would the government want this personal information on American citizens?"

Back inside the Capitol, House and Senate Republicans continued to hammer the Obama administration, unappeased by President Obama's firing of the tax agency's acting head on Wednesday.

Senate Minority Leader Mitch McConnell, R-Ky., dismissed Obama's ouster of Steven Miller as a "symbolic" gesture since the IRS head was already planning to leave next month. McConnell said investigations must continue to determine why the targeting was not disclosed earlier.

Republican members of the Senate Finance Committee called for an additional investigation into reports that the IRS last year illegally provided a confidential list of donors to the conservative National Organization for Marriage to the liberal Human Rights Campaign, which published the list on its website.

The Human Rights Campaign at the time was headed by a top Obama campaign operative, and one of the listed donors was Mitt Romney, Obama's Republican opponent.

"There are allegations here that someone at the IRS committed a very serious crime that had the effect of chilling the speech of a political organization that happened to be on the wrong side of the current administration," McConnell said.

House Speaker John Boehner, R-Ohio, pledged to "stop at nothing" to determine which high-ranking IRS officials gave the order to target conservative groups. On Wednesday, IRS officials told senators in a closed-door meeting that entire episode was the work of just two "rogue" employees based in the agency's Cincinnati office -- a claim Repubicans dismissed.

"Somebody made a decision to do this," said Boehner, "and I doubt that it was some low-level employees in the Cincinnati field office."