WASHINGTON -- Internal Revenue Service officials are not fully cooperating with efforts to learn who is responsible for targeting conservative groups, lawmakers learned Wednesday during the third and most tense, dramatic hearing on the scandal.

First, the director of the IRS division that oversees tax-exempt organizations at the heart of the scandal invoked her constitutional Fifth Amendment right and refused to answer questions during an appearance before the House Committee on Oversight and Government Reform.

Then, a Treasury Department inspector general told the committee that IRS employees in the Cincinnati office that handled applications for tax-exempt designations were not being fully cooperative in the investigation. He said the IRS employees have stymied efforts to learn who developed the controversial questions asked of conservative groups.

The inability to find out more about who authorized the special scrutiny of conservative groups frustrated lawmakers striving to find out more even as the IRS launches its own 30-day inquiry and the Justice Department is starting a criminal investigation. At the same time Wednesday, the White House press secretary acknowledged that there have been “legitimate” criticisms about the way he has explained what the administration knew about the scandal.

The most dramatic moment of the day came when Lois Lerner, a career government lawyer and longtime IRS official, appeared before the House committee.

“I have not done anything wrong,” Lerner said in an opening statement. “I have not broken any law. I have not violated any IRS rules and regulations, and I have not provided false information to this or any other congressional committee.”

She then informed the committee she was invoking her right to not answer questions.

“One of the basic functions of the Fifth Amendment is to protect innocent individuals,” Lerner said, repeating her innocence amid allegations from lawmakers that she misled Congress.

“You don’t get to tell your side of the story and then not answer questions,” insisted Rep. Trey Gowdy, R-S.C., furious and arguing to Chairman Darrell Issa, R-Calif., that Lerner effectively waived her rights by first issuing a statement.

Issa eventually excused Lerner and she left by a side door.

Later, J. Russell George, Treasury’s inspector general for the Office of Tax Administration, told the panel that he has not been able to find out everything he wants from IRS employees.

"We have had some difficulty in terms of getting clarity from some of the employees we’ve interviewed," he said, adding that further inquiries could make those employees more forthcoming.

Before going silent Wednesday, Lerner gave her work history, noting that she had worked at the Justice Department and the Federal Election Commission before joining the IRS. She joined the tax-exempt division in 2001 and was promoted to head the division in 2006, overseeing roughly 1.6 million tax-exempt organizations with a staff of 900 employees and a budget of almost $100 million.

She’s a key figure in the scandal because George’s audit found that she apparently had knowledge of IRS employees flagging conservative groups for special scrutiny when they applied for tax-exempt status as social welfare groups. The audit said she instructed the Cincinnati office to back off the inappropriate code words used to flag applications from conservative groups.