The F.B.I. told a federal magistrate judge in Manhattan in a sealed affidavit shortly before Election Day that there was probable cause to believe that emails belonging to Huma Abedin, a top aide to Hillary Clinton, contained classified information, according to a redacted copy of the affidavit.

The affidavit, which was unsealed on Tuesday along with related materials in Federal District Court, was filed on Oct. 30 in support of the F.B.I.’s request for a warrant to search a laptop containing the emails.

The government appears to have based its warrant request on the theory that because Mrs. Clinton and Ms. Abedin had exchanged many classified emails using Mrs. Clinton’s personal server, there was likely to be classified material in the additional emails discovered on the laptop, as well.

The affidavit, in a redacted reference that a law enforcement official said was to Ms. Abedin, noted that she typically communicated with Mrs. Clinton’s email account on a daily basis, and that the bureau had found more than 4,000 work-related emails between them from 2009 to 2013.