WASHINGTON — An aide to House Majority Leader Eric Cantor says the Virginia congressman first heard about CIA Director David Petraeus’ extramarital affair on Oct. 27 from an FBI source he didn’t know.

Communications director Rory Cooper told The Associated Press today that Cantor notified the FBI’s chief of staff of the conversation, but did not tell anyone else because he did not know whether the information from an unknown source was credible. Petraeus resigned last week as the nation’s top spy because of the affair with his biographer, Paula Broadwell.

The spokesman said the Oct. 27 conversation was arranged by Rep. Dave Reichert, a Washington Republican. Reichert had initially received a tip from an FBI source who was a colleague of the bureau employee who called Cantor.

The day after the late-October call, Cooper said, Cantor conferred with his chief of staff, Steve Stombres, and Richard Cullen, a former attorney general of Virginia who also served as U.S. attorney for the Eastern District of Virginia.

Cantor decided after those conversations to call the FBI, but couldn’t do so until Oct. 31 because the government was closed due to Superstorm Sandy.