WASHINGTON — Defiant before skeptical Republicans, the head of the IRS refused to apologize Friday for lost emails that might shed light on the tax agency’s targeting of tea party and other groups before the 2010 and 2012 elections.

Instead, Commissioner John Koskinen accused the chairman of a powerful House committee 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.”

“I have a long career. That’s the first time anybody has said they do not believe me,” said Koskinen, who came out of retirement in December to take over the IRS. Previously, he served in other positions under Presidents Bill Clinton and George W. Bush.

The hearing showed that emotions are running hotter than ever in the dispute over the IRS and political fundraising.