Miller said the IRS has had trouble keeping up with the flood of 70,000 tax-exempt applications it has received in recent years, and asked for money to hire more examiners.
Several Republicans responded that the IRS should instead be shrunk.
Tea Party groups investigated by the IRS say the tax agency made unusually extensive demands, such as asking the groups to provide social-media posts and lists of books that members had read, and tell agents whether any members of the group planned to run for public office in the future.
Republicans have vowed to find out who was involved, but Miller did not provide much of a road map.
“Who is responsible for targeting these individuals?” asked Representative Kevin Brady, a Texas Republican.
“I don’t have names for you,” Miller responded.
Republicans accused him of dodging their questions.
“I’m hearing, ‘I don’t know, I don’t remember, I don’t recall, I don’t believe,’” said Representative Dave Reichert of Washington. “You don’t even know who investigated the case, but yet you say it was investigated.”
Two other congressional committees will hold IRS hearings next week.