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Although he will wait to launch a probe until he reviews the independent inspector general’s report, Mr. Baucus said, “the IRS should be prepared for a full investigation into this matter by the Senate Finance Committee.”

“The American people have questions for the IRS and I intend to get answers,” he said. “The IRS will now be the ones put under additional scrutiny.”

Sen. Joe Manchin III, West Virginia Democrat, called the IRS actions “unacceptable and un-American.” He called on Mr. Obama to “immediately condemn this attack on our values, find those individuals in his administration who are responsible and fire them.”

Lois Lerner, who heads the IRS division that oversees tax-exempt groups, on Friday apologized for the “inappropriate” practice and said the cases were initiated by low-level workers in Cincinnati and were not motivated by political bias.

But some senior IRS officials knew agents were targeting tea party groups as early as June 2011, according to an inspector general’s timeline of events obtained by The Washington Times.

Mr. Shulman denied that the agency was targeting conservative groups when he testified on Capitol Hill in March 2012. It’s unclear from the timeline when, or whether, the former commissioner knew his agency was targeting conservative groups.

Mr. Miller failed to tell Congress that tea party groups were being inappropriately targeted, even after he had been briefed on the matter, The Associated Press reported.

The IRS said Monday that Mr. Miller was first informed on May, 3, 2012, that applications for tax-exempt status by tea party groups were singled out for extra, sometimes burdensome, scrutiny.

But after House Ways and Means Committee member Charles Boustany, Louisiana Republican, raised concerns with the IRS about complaints that tea party groups were being harassed, Mr. Shulman wrote to the panel on June 15, 2012, to explain the process of reviewing applications for tax-exempt status without mentioning the controversy.