Comey said that on October 27, the Federal Bureau of Investigation team that had previously investigated Clinton's use of a private email server while serving as secretary of state came to him with metadata they could see on a laptop belonging to former Rep. Anthony Weiner.
Comey said that when he first spoke about the Clinton case, in July 2016, he did so because the investigation had been closed, no charges had been made, and he felt that if the Department of Justice in the Obama administration made the announcement it would lose credibility.
Comey says he was faced with two options: either hide the investigation until after the November 8 vote, or inform Congress.
He saw two doors: One labeled "speak" was bad, but the other labeled "conceal" was "catastrophic".
The FBI, which is supposed to remain politically neutral, said a few days later that the new emails did not change an earlier decision not to recommend criminal charges against Mrs Clinton, but many Democrats believe the political damage was already done.
President Donald Trump has confidence in FBI Director James Comey, White House spokesman Sean Spicer said Wednesday, despite his tweet Tuesday saying that he had given Hillary Clinton "a free pass for many bad deeds" during the election.More news: United Airlines, doctor dragged off flight settle lawsuit
"It makes me mildly nauseous to think that we might have had some impact on the election, but honestly, it wouldn't change the decision", Comey told the committee.
Comey also made it clear in the hearing that Russian Federation, which US intelligence agencies concluded hacked Democratic operatives' emails to aid Trump's campaign, remains a threat.
He says he believes a large portion of WikiLeaks' work "has nothing to do with legitimate newsgathering".
"I want that inspection, I want my story told, " he said.
FBI agents had seized the computer of former congressman Anthony Weiner, Abedin's separated husband, as part of separate investigation into a possible sexting scandal involving a minor.
Comey said that it was a good question, seemingly frustrating Grassley even more.More news: More than 230000 British Columbians cast ballots in advance polls
Comey said if he finds out people leaked information about FBI investigations, "there will be severe consequences".
Thankfully at least one questioner - Senator Patrick Leahy of Vermont - didn't let go of the fact that Comey publicly spoke on the Clinton emails - but NOT on the Trump/Russia investigation that was open at the time.
Still, when she asks her loaded question, the intensity and clarity of Comey's answer was probably not what she was hoping to hear.
Comey added that not notifying Congress simply because it could influence the election would politicise the agency and result in the "death of the FBI's independent institution in America". Their last meeting with Comey, a public session in March, gave the clearest picture yet of what is at stake. Patrick Leahy of Vermont, Comey maintained that he did not treat disclosures about investigations into Clinton's emails differently than potential connections between the Trump campaign and Russian Federation. "It's so hard for people to see that", Comey said, adding later, "I don't have any regrets".
Following those remarks, Trump criticized Comey the night before the hearing.More news: Trump to meet Australian Prime Minister Turnbull in New York