The F.B.I. is investigating illicit text messages that Mr. Weiner, a former Democratic congressman from New York, sent to a 15-year-old girl in North Carolina. The bureau told Congress on Friday that it had uncovered new emails related to the Clinton case — one federal official said they numbered in the thousands — 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."

Mr. Comey said the F.B.I. was taking steps to "determine whether they contain classified information, as well as to assess their importance to our investigation." He said he did not know how long it would take to review the emails, or whether the new information was significant.

Mr. Trump has fallen behind Mrs. Clinton in most national polls and in many key states. Polls have been tightening in recent days, however, amid the daily release of hacked Clinton campaign emails published by WikiLeaks.

Mr. Trump seized on the F.B.I. action on Friday at a rally in New Hampshire. To cheers of "lock her up" from his supporters, Mr. Trump said: "Hillary Clinton's corruption is on a scale we have never seen before. We must not let her take her criminal scheme into the Oval Office."