Clinton was projected by nearly every national public opinion poll as the heavy favorite going into Tuesday's race. Instead, Republican Donald Trump won the election, shocking many throughout the nation and prompting widespread protests.
Clinton has kept a low profile since her defeat after delivering her concession speech on Wednesday morning.
Clinton told her supporters on Saturday that her team had drafted a memo that looked at the changing opinion polls leading up to the election and that the letter from Comey proved to be a turning point. She said Comey's decision to go public with the renewed examination of her email server had caused an erosion of support in the upper Midwest, according to three people familiar with the call.
Clinton lost in Wisconsin, the first time since 1984 that the state favored the Republican candidate in a presidential election. Although the final result in Michigan has still not been tallied, it is leaning Republican, in a state that last favored the Republican nominee in 1988.
Comey sent a letter to Congress only days before the election announcing that he was reinstating an investigation into whether Clinton mishandled classified information when she used a private email server while secretary of state from 2009 to 2012.
Comey announced a week later that he had reviewed emails and continued to believe she should not be prosecuted, but the political damage was already done.
Clinton told donors that Trump was able to seize on both of Comey's announcements and use them to attack her, according to two participants on the call.
While the second letter cleared her of wrongdoing, Clinton said that it reinforced to Trump's supporters that the system was rigged in her favor and motivated them to mobilize on Election Day.
The memo prepared by Clinton's campaign, a copy of which was seen by Reuters, said voters who decided which candidate to support in the last week were more likely to support Trump than Clinton.
"In the end, late breaking developments in the race proved one hurdle too many for us to overcome," the memo concludes.
A spokesperson for the FBI could not immediately be reached for comment.
On the phone call, Dennis Chang, who served as Clinton's finance chair, said her campaign and the national party had raised more than $900 million from more than 3 million individual donors, according to the two participants who spoke to Reuters.
TRUMP: 'RESTRAINED' TWITTER PRESENCE
As Clinton gave her account to donors, Trump hunkered down at Trump Tower with members of the transition team announced on Friday and tasked with selecting the 15 Cabinet posts and thousands of political appointment jobs.
Kellyanne Conway, who served as his campaign manager, said on Saturday that the an announcement of a new chief of staff is "imminent." Two candidates whose names have surfaced as contenders for the top White House job are campaign CEO Steve Bannon and Republican National Committee Chairman Reince Priebus.
Trump will deliver a speech about his plans moving forward in the coming days and may undertake a national victory tour, Conway said, without providing further details. He will be sworn in on January 20.
The president-elect plans to keep his communication channels open. In an interview with CBS' "60 Minutes" that will air on Sunday, Trump said he isn't ready to give up his Twitter account, where he routinely posted controversial statements during the campaign that unleashed harsh criticism.
"I'm going to be very restrained, if I use it at all, I'm going to be very restrained," Trump said.