Friday's hearing is the first of what are expected to be many on the subject by congressional panels. Underscoring the seriousness of the episode, Miller was sworn in as a witness, an unusual step for the Ways and Means panel and one that could put Miller in jeopardy if he is later shown to have misled lawmakers with his testimony.
When the hearing ended after nearly four hours, Camp said, "I promise the American people, this investigation has just begun."
Levin said that the IRS's mistreatment of conservative groups meant the agency "completely failed the American people." He said Lois Lerner, who heads the IRS division that makes decisions about tax-exempt groups, should be "relieved of her duties."
Miller said the IRS struggled to efficiently handle growing numbers of applications for tax-exempt status.
The agency has said between 2008 and 2012, the number of groups applying for tax-exempt status as so-called social welfare groups more than doubled. Along with that was an increase in complaints that such groups were largely engaging in electoral politics, which is not supposed to be their primary activity.
"I do not believe partisanship motivated the people" at the IRS who engaged in the harsher screening for conservative groups, Miller said.
In recent months, Republicans on the Ways and Means panel had repeatedly asked the IRS about complaints from conservative groups that their applications were being treated unfairly.
On Friday, numerous Republicans wanted to know why Miller and others never told them the groups were being targeted, even after May 2012, when the IRS has said Miller was briefed on the practice. Miller was previously a deputy commissioner whose portfolio included the unit that made decisions about tax-exempt status.