Irate lawmakers vowed Monday to hold congressional hearings on the Internal Revenue Service after the tax agency admitted that it closely scrutinized the tax-exempt status of conservative groups that opposed President Obama and other Democrats.

The Republican-run House Ways and Means Committee and the Oversight and Government Reform Committee as well as the Democrat-controlled Senate Finance Committee all plan to investigate the IRS for targeting groups aligned with the Tea Party or "patriot" movement as well as other tax-exempt groups the agency considered hostile to the government.

The IRS also acknowledged Monday that its acting commissioner, Steven T. Miller, who had been briefed on the screening of conservative groups last May, had repeatedly failed to tell Congress that Tea Party groups and other were being inappropriately targeted, even when asked about it by lawmakers.

"Targeting any group based on its political stance is completely inappropriate," Senate Majority Leader Harry Reid, D-Nev., said Monday. "We need to get to the bottom of what happened here."

Lawmakers immediately introduced legislation aimed at stopping the IRS from targeting political groups.

Rep. Mike Turner, R-Ohio, introduced a measure that would make it a crime for IRS employees to discriminate against a group based on political or religious affiliation. Sen. Marco Rubio, R-Fla., will introduce similar legislation in the Senate.

For some lawmakers, the IRS' admission on Friday was reminiscent of the "enemies list" created by former President Richard Nixon to track people he didn't like.

"This is the United States of America, where the First Amendment protects our right to organize and speak up and speak out, and it's shocking to learn that the IRS arbitrarily targeted any peaceful political organization for ideological reasons," said Sen. Lamar Alexander, R-Tenn.

While offering their own criticisms of the IRS, Democrats made every attempt to tamp down any suggestion that the independent agency's actions were connected to the Obama White House.

"We need to get to the bottom of what happened and the inspector general will get to the bottom of this," Reid said. "In the meantime, no one should jump to conclusions."

Lois Lerner, who runs the IRS office that overseas tax-exempt groups, placed blame for the debacle on low-level employees in the agency's Cincinnati office, insisting their actions were not politically motivated. A soon-to-be-released inspector general report on the IRS, which first identified its scrutiny of conservative groups, said "senior Internal Revenue Service officials" were aware of the scrutiny as far back as 2011, the Associated Press reported.

The IRS scrutiny of conservative groups was discovered during an inspector general investigation that was sparked by an inquiry from House Oversight and Government Reform Committee Chairman Darrell Issa, R-Calif.

"The committee will aggressively follow up on the IG report and hold responsible officials accountable for this political retaliation," Issa said Sunday on "Meet the Press."

The IRS began looking into the conservative groups after the U.S. Supreme Court changed campaign finance laws and Senate Finance Committee Chairman Max Baucus, D-Mont., asked the agency to examine whether any groups were violating their tax-exempt status by getting involved in political campaigns.

The letter does not request that any particular group be targeted and Baucus expressed anger Monday at the revelation that the IRS used such criteria to scrutinize the groups, promising to investigate the matter further.

"The IRS will now be the ones put under additional scrutiny," Baucus said.