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Ms. Lerner apologized at a May 10 event with the American Bar Association for burdening the conservative groups from early 2010 to May 2012, an admission that set off a firestorm on Capitol Hill.

The announcement — staged through a prearranged question from the audience — was made days before the inspector general released the audit that confirmed Republican lawmakers’ suspicions in 2012 that conservative groups had been singled out.

Wednesday’s hearing was the third since Ms. Lerner revealed the inappropriate activity two weeks ago, and it underscored the remarkable, sustained anger at the tax agency from both sides of the aisle.

An inspector general said the IRS gave special scrutiny to applications for tax-exempt status filed by groups that had “tea party,” “patriot” or “9/12” in their names. The IRS is holding up dozens of those applications, some of which were filed three or four years ago.

IRS officials, including Mr. Shulman, previously told Congress that there was no effort to target conservative groups. On Wednesday, Mr. Shulman said he told the truth as he believed it at the time.

He testified that he had “some of the facts, not all of the facts,” and that he did not want, as a political appointee, to give the impression that he was interfering with the inspector general’s review of the situation.

“If there’s someone wielding a knife in the parking lot, are you going to call the inspector general?” Mr. Gowdy said. “Are you going to wait until his or her investigation’s over before you stop it?”

Mr. Shulman reiterated his point about the inspector general and said he was under the impression last year that the offending behavior was being stopped.

Lawmakers said that strained belief and that once Mr. Shulman learned that the inspector general was looking into the reports, he should have alerted Congress, which was clearly interested. Lawmakers said they sent 132 letters to the IRS asking about the situation.

Congress was misled,” Mr. Issa said. “The American people were misled.”

Lawmakers also chastised Mr. Shulman for failing to take any action within his agency.

Rep. Tammy Duckworth, Illinois Democrat and a veteran who lost both legs in combat, said there are “25-year-old buck sergeants and second lieutenants who know you can delegate authority, but you can never delegate responsibility and that you’re always responsible for the performance, the training, the actions of the men and women under you.”

Mr. Issa also took the inspector general to task for the length of time it took for him to release his findings, or at least inform his committee of any wrongdoing.

Deputy Treasury Secretary Neal S. Wolin addressed the scandal before lawmakers for the first time. He deemed the political targeting “absolutely unacceptable and inexcusable,” but denied responsibility or knowledge of the actions that led to the scandal.

Treasury Secretary Jack Lew and President Obama took immediate action by demanding and accepting the acting commissioner’s resignation, he said, and Mr. Lew ordered Daniel Werfel, the newly appointed commissioner, to conduct a thorough review how the agency handles applications for tax-exempt status.