“We need to hear how the F.B.I. will regain that faith and trust,” she said. “We need straightforward answers to our questions, and we want to hear how you’re going to lead the F.B.I. going forward. We never, ever want anything like this to happen again.”

She demanded to know why his treatment of the Clinton investigation had been so “dramatically different” from his treatment of an investigation into Russian efforts to meddle in the election.

Mr. Comey rejected her claim.

He said that the F.B.I. had confirmed the existence of the investigation into Mrs. Clinton’s emails months after it began, and said nothing more until after it was closed. Similarly, Mr. Comey said, the F.B.I. revealed its Russia investigation months after it was opened in the summer, and only after it had been widely reported in the news media. And, as with the Clinton inquiry while it was still underway, the bureau has refused to talk about what it has found with regard to Russia.

“We’re not going to say another peep about it until we’re done,” Mr. Comey said, acknowledging that the inquiry was continuing. “And I don’t know what will be said when we’re done, but that’s the way we handled the Clinton investigation, as well.”

The tone of the opening statements from both Ms. Feinstein and the Republican chairman of the committee, Senator Charles E. Grassley of Iowa, made clear that they wanted answers from Mr. Comey on a number of issues: Mrs. Clinton’s emails, the Russia investigation, leaks to the news media and the use of wiretapping as an investigative tool.

“We need the F.B.I. to be accountable because we need the F.B.I. to be effective,” Mr. Grassley said.