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The FBI's surprise decision to re-examine Hillary Clinton's use of a private email server before voters went to the polls cost her the election, the "heartbroken" Democratic nominee said Saturday, a source told NBC News.

The letter about the email inquiry — sent from FBI Director James Comey to Congress 11 days before the vote — raised doubts that helped to stop the campaign's momentum at a critical time, Clinton said in a farewell conference call with donors. A source who was on the call, which was reported by The Washington Post and others, confirmed Clinton's remarks to NBC News.

The source added that Clinton acknowledged that other factors contributed to her failed bid.

But Comey's closure of the inquiry just two days before the election on Tuesday boosted undecided voters in Donald Trump's favor even further, Clinton reportedly said.

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It was the first time Clinton spoke extensively about her loss.

On Wednesday, the day after Trump's stunning upset, Clinton gave a gracious concession speech in which she urged Americans to give Trump "a chance to lead" and told women and girls to never give up on their dreams, even if they suffer setbacks.

But she didn't delve into what she felt had led to her defeat.

She also didn't mention the FBI inquiry during an appearance Friday night at her Brooklyn, New York, campaign headquarters where she thanked staffers.

Comey's letter revealed that a newly discovered batch of emails had prompted him to take another look at the email investigation that had dogged the former secretary of state throughout her candidacy. For the following eight days, Americans waited as the FBI sifted through 450,000 emails — only for Comey to then announce that the bureau wouldn't reverse its initial decision from July that there was no evidence of criminality on Clinton's part.

Related: Who Is James Comey? He's Dealt With Minefields Before

While Comey has donated to past GOP presidential candidates and was a registered Republican until recently, Clinton did not mention his party affiliations on Saturday.

Trump, for his part, latched onto his claims that the election was "rigged" after the speedy conclusion from the FBI that nothing in the new emails merited charges against Clinton.

"Right now, she is being protected by a rigged system. It's a totally rigged system," Trump had said, adding that it wasn't possible to review that many emails in such a short period of time. Computer experts — even former CIA employee Edward Snowden, who leaked classified information from the government — have said otherwise.

By all accounts, Clinton appeared to be on the path to winning the election. Polls taken after the presidential debates showed her emerging as the stronger candidate in voters' minds, and an NBC/WSJ poll taken days before the election showed her up by 4 points, even after the FBI inquiry had come and gone.

Ultimately, Clinton said Saturday, Comey put a damper on the campaign. She also admitted that she was devastated by her loss and said she was "heartbroken."