The IRS official overseeing the division that was inappropriately targeting conservative groups said she will refuse to answer lawmakers' questions during an appearance before a House committee Wednesday, a day after another top official at the agency acknowledged that he knew the targeting was happening but chose to keep quiet about it.

Lois Lerner, head of the Internal Revenue Service's Exempt Organizations Division, informed the House Oversight and Government Reform Committee that she will invoke Fifth Amendment protections against self-incrimination during her appearance Wednesday. Lerner was the first to acknowledge publicly that the IRS was targeting conservative groups seeking tax-exempt status, though she blamed the entire incident on a few rogue employees.

A committee spokesman said the panel's chairman, Rep. Darrell Issa, R-Calif., is urging Lerner to change her mind.

"The committee has a constitutional obligation to conduct oversight," the aide said. "Chairman Issa remains hopeful that she will ultimately decide to testify [Wednesday] about her knowledge of outrageous IRS targeting of Americans for their political beliefs."

Douglas Schulman, a former IRS commissioner, on Tuesday became the latest executive branch official called to explain why the IRS was targeting Tea Party and other conservative groups that intended to play a part in the 2012 election. Republicans say the targeting was politically motivated. Democrats insist changes in campaign finance laws caused a flood of applications for tax-exempt status that simply overwhelmed the IRS.

Schulman, IRS commissioner from 2008 until last November, told the Senate Finance Committee that he first learned in the spring of 2012 that his employees were delaying approval of tax-exempt status for conservative groups, marking those applications with "Be On The Look Out." But Schulman said he didn't tell anyone at Treasury, which oversees the IRS, because "this is not the kind of information that should leave the IRS."

Schulman appeared at the third congressional hearing so far on the IRS alongside J. Russell George, the inspector general who first disclosed that the targeting began as early as 2011 and who told the committee he had informed Treasury officials and a congressional commmittee about his investigation in 2012.

Even as the Obama administration tried to put the budding scandal behind it, White House spokesman Jay Carney on Tuesday revealed that more senior administration officials than previously acknowledged knew about the IRS targeting and probe. The White House says President Obama was not informed of either at that time.

Committee members reacted angrily when Schulman admitted he had heard of the probe just a week after he appeared on Capitol Hill to deny that the IRS were targeting conservatives. Lawmakers first asked Schulman about it after local Tea Party officials told them they were being harrassed by the IRS.

"When you say one thing before the committee and then find out it's another," said Sen. Orrin Hatch, the committee's top Republican, "I think you have an obligation to let our committee know about it."

Lawmakers insist that they will find out who was responsible for starting the targeting process for Obama's political opponents. Until then, George blamed "gross mismanagement" within the agency.

"Why wasn't more firm action taken by people, either the commissioner himself or by people at the top?" asked Senate Finance Committee Chairman Max Baucus, D-Mont. "It's outrageous. Any person can figure out this was unacceptable conduct."