The director of the FBI has said he feels "mildly nauseous" at the thought he affected the outcome of last year's presidential election.
James Comey was speaking publicly for the first time about his decision to reveal that the bureau was re-opening the investigation into Hillary Clinton over her private email server.
The bombshell revelation came on 28 October, just 11 days before the election which led to Mrs Clinton's shock defeat to Donald Trump.
On Tuesday, Mrs Clinton - who was subsequently cleared of any wrongdoing - claimed the incident had been a factor in her defeat. "If the election had been on 27 October, I'd be your president," she said.
But Mr Comey told the Senate Judiciary Committee that it would have been worse to have concealed his decision - and that he would make the same choice again.
He said: "It makes me mildly nauseous to think we might have had an impact on the election but honestly it wouldn't change the decision."
Mr Comey re-opened the investigation into Mrs Clinton's use of a private email server while secretary of state - months after declaring it completed - following the discovery of emails on the laptop of former congressman Anthony Wiener.
He told the hearing: "To restart, in a huge way, potentially finding her emails that would reflect on her intent from the beginning and not speak about it would require an act of concealment, in my view.
"Speak would be really bad, there's an election in 11 days. Lordy, that would be really bad. Concealing, in my view, would be catastrophic. Not just to the FBI but well beyond and honestly between really bad and catastrophic, I said to my team, we've got to walk into the world of really bad.
"Look, this was terrible. Anybody that disagrees with me has to come back to 28 October and stare at this and tell me, what you would do?"
Mr Comey has been accused of swaying the election in Mr Trump's favour with his decision.
His comments on Capitol Hill displayed rare public emotion from a director of America's domestic intelligence and security service.
He said: "I've lived my entire career by the tradition that, if you can possibly avoid it, you avoid any action in the run-up to an election that might have impact, whether it's a dogcatcher election or president of the United States.
"Even in hindsight, and this has been one of the world's most painful experiences, I would make the same decision."
The director also repeated his confirmation that the bureau was investigating possible connections between the Trump campaign and Russia.
He denied claims by Democrat senators that he had displayed a double standard on treatment of the two candidates and would add nothing more on the Trump investigation.
"I would expect we're not going to say another peep about it until we're done, and I don't know what will be said when we're done. That's the way we handled the Clinton investigation as well."