Hillary Clinton on Saturday cast blame for her surprise election loss on the announcement by the F.B.I. director, James B. Comey, days before the election that he had revived the inquiry into her use of a private email server.

In her most extensive remarks since she conceded the race to Donald J. Trump early Wednesday, Mrs. Clinton told donors on a 30-minute conference call that Mr. Comey’s decision to send a letter to Congress about the inquiry 11 days before Election Day had thrust the controversy back into the news and had prevented her from ending the campaign with an optimistic closing argument.

“There are lots of reasons why an election like this is not successful,” Mrs. Clinton said, according to a donor who relayed the remarks. But, she added, “our analysis is that Comey’s letter raising doubts that were groundless, baseless, proven to be, stopped our momentum.”

Mrs. Clinton said a second letter from Mr. Comey, clearing her once again, which came two days before Election Day, had been even more damaging. In that letter, Mr. Comey said an examination of a new trove of emails, which had been found on the computer of Anthony D. Weiner, the estranged husband of one of her top aides, had not caused him to change his earlier conclusion that Mrs. Clinton should face no charges over her handling of classified information.