The Internal Revenue Service said Friday it has lost a trove of emails to and from a central figure in the agency’s tea party controversy, sparking outrage from congressional investigators who have been probing the agency for more than a year.

The IRS told Congress Friday it cannot locate many of Lois Lerner’s emails before 2011 because her computer crashed during the summer of that year.

Lerner headed the IRS division that processed applications for tax-exempt status. The IRS acknowledged last year that agents had improperly scrutinized applications for tax-exempt status by tea party and other conservative groups.

“The fact that I am just learning about this, over a year into the investigation, is completely unacceptable and now calls into question the credibility of the IRS’s response to congressional inquiries,” said Rep. Dave Camp, R-Mich., chairman of the House Ways and Means Committee. “There needs to be an immediate investigation and forensic audit by Department of Justice as well as the inspector general.”

The Ways and Means Committee is one of three congressional committees investigating the IRS over its handling of tea party applications from 2010 to 2012. The Justice Department and the IRS inspector general are also investigating.

Congressional investigators have shown that IRS officials in Washington were closely involved in the handling of tea party applications, many of which languished for more than a year without action. But so far, they have not publicly produced evidence that anyone outside the agency directed the targeting or even knew about it.

If anyone outside the agency was involved, investigators were hoping for clues in Lerner’s emails.

The IRS was able to generate 24,000 Lerner emails from 2009 to 2011 because Lerner had copied in other IRS employees. The agency said it pieced together the emails from the computers of 82 other IRS employees.

But an untold number are gone. Camp’s office said the missing emails are mainly ones to and from people outside the IRS, “such as the White House, Treasury, Department of Justice, FEC, or Democrat offices.”

The IRS said in a statement that it is cooperating with congressional investigations, spending nearly $10 million to produce more than 750,000 documents.

Overall, the IRS said it is producing a total of 67,000 emails to and from Lerner, covering the period from 2009 to 2013.

“The IRS is committed to working with Congress,” the IRS said in a statement.

Lerner has emerged as a key figure in the tea party probe. In May 2013, she was the first IRS official to publicly acknowledge that agents had improperly scrutinized applications.