The conclusion from Comey provided one last twist to the 2016 presidential campaign and came just two days before Clinton will face Republican Donald Trump on Election Day.
Comey had upended the campaign just over a week ago, when he alerted Congress that new emails had been located that were related to Clinton's use of a private server as secretary of state.
The emails came from a laptop owned by Weiner, the estranged husband of top Clinton aide Huma Abedin, and had been seized by the FBI on October 3 as part of an investigation into lewd text messages Weiner is alleged to have sent to a 15-year-old girl.
Comey had come under fire for inserting the FBI into the campaign's final days with the announcement. Department of Justice policy discourages the agency from taking steps in days before an election, to avoid the perception that the FBI is trying to the influence the outcome of the vote.
A spokesman for the FBI declined to comment beyond Comey's letter.