Federal law enforcement officials said Friday that the new emails uncovered in the closed investigation into Hillary Clinton’s use of a private email server were discovered after the F.B.I. seized electronic devices belonging to Huma Abedin, an aide to Mrs. Clinton, and her husband, Anthony Weiner.

The F.B.I. told Congress that it had uncovered new emails related to the closed investigation into whether Mrs. Clinton or her aides had mishandled classified information, potentially reigniting an issue that has weighed on the presidential campaign and offering a lifeline to Donald J. Trump less than two weeks before the election.

In a letter to Congress, the F.B.I. director, James B. Comey, said that emails had surfaced in an unrelated case, and that they “appear to be pertinent to the investigation.”