WASHINGTON - Lois Lerner, the Internal Revenue Service official at the center of a controversy over the tax agency's targeting of tea party groups, will retire, according to Congressional sources.

Lerner had been on paid administrative leave at the agency since May, after she acknowledged at an American Bar Association conference that the IRS inappropriately scrutinized conservative groups for years. As the director of the Exempt Organizations group, Lerner supervised the unit responsible for the targeting.

But she infuriated Republican lawmakers by invoking her Fifth Amendment rights and refusing to testify at a Congressional hearing into the matter.

Republican lawmakers, who have called for Lerner to be fired for months, said her retirement doesn't put to rest questions about her involvement in the targeting controversy.

"Lois Lerner's exit from the IRS does not alter the Oversight Committee's interest in understanding why applicants for tax exempt status were targeted and inappropriately treated because of their political beliefs," House Oversight Committee Chairman Darrell Issa said in a statement. "We still don't know why Lois Lerner, as a senior IRS official, had such a personal interest in directing scrutiny and why she denied improper conduct to Congress.

"Her departure does not answer these questions or diminish the Committee's interest in hearing her testimony," he added.

Democrats, however, have insisted that none of the inquiries into the matter found that there was any political motivation for targeting conservative groups.

House Ways and Means Committee ranking member Sander Levin, D-Mich., cautioned his Republican colleagues against continuing to suggest that the IRS's actions were driven by political considerations.

"Lois Lerner is being held responsible for her gross mismanagement of the IRS tax-exempt division, which led to improper handling of applications for tax-exempt status, whether conservative and progressive," Levin said in a statement. "As has been the case in all aspects of the current IRS investigation, the IRS internal Review Board found no evidence of political bias in her neglect of duties."

The IRS confirmed that Lerner has retired, but declined to comment any further on "individual employee matters," citing federal privacy rules.

In a statement, the agency said it has made progress in fixing its problems and would continue to cooperate with Congress.