"There are lots of reasons why an election like this is not successful. Our analysis is that Comey's letter raising doubts that were groundless, baseless, proven to be, stopped our momentum," Clinton told Democratic fundraisers and donors during a conference call yesterday.
The conversation, which lasted about 30 minutes, was not open to the press.
Clinton said her campaign was winning until FBI Director Comey wrote a letter to the Congressional leadership that they have reopened the investigations into the email scandal.
Then, on November 6, two days before the election, Comey said in a second letter that he was standing by his original assessment - that Clinton should not face criminal charges.
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"The second letter, even that gave her the clean chit, hit her more as this energised Trump's supporters. Although she got more popular votes, Trump was declared the winner as he grabbed more than 270 electoral college votes, required to win the presidential elections," said the 69-year-old former Secretary of State.
"We were once again up in all but two of the battleground states, and we were up considerably in some that we ended up losing. And we were feeling like we had to put it back together," Clinton said.
The FBI had investigated Clinton using a private email server system while secretary of state.
In an internal campaign memo, Clinton's campaign manager Robby Mook claimed that she was poised to win the elections, but the two letters from FBI changed the scenario.
He also appeared to blame the third party candidate.
"It is worth mentioning that Jill Stein alone got 130K votes in those States - and though her votes don't distribute perfectly to cover the margin across the three States, it is an important reminder of 3rd party votes," he said.
"We believe that we lost this election in the last week. Comey's letter in the last 11 days of the election both helped depress our turnout and drove away some of our critical support among college-educated white voters.