WASHINGTON (AP) — A defiant head of the IRS is refusing to apologize for lost emails that might shed light on the tax agency's targeting of tea party and other groups.

Internal Revenue Service (IRS) Commissioner John Koskinen testifies during a hearing before the House Ways and Means Committee (Alex Wong/Getty Images)

Commissioner John Koskinen testified today before the House Ways and Means Committee, at one point accusing its chairman, Dave Camp, of misleading the public by making false statements based on incomplete information.

The contentious back-and-forth didn't end there. Later in the hearing, Rep. Paul Ryan, the Republicans' vice presidential candidate two years ago, told Koskinen bluntly that "nobody believes you."

Koskinen was called to testify a week after the IRS disclosed that it had lost an untold number of emails to and from Lois Lerner. Lerner headed the division that processes applications for tax-exempt status during a time when, the IRS has acknowledged, agents improperly scrutinized applications from tea party and other conservative groups. The IRS says it lost Lerner's emails when her computer crashed in June 2011.

The IRS commissioner dismissed Camp's call for a special prosecutor to investigate, saying it would be "a monumental waste of taxpayer funds."