A spokeswoman for Ensign, Rebecca Fisher, would not say Tuesday whether the Republican lawmaker talked with the FBI.
Ensign acknowledged the affair with Cynthia Hampton in June. He also helped her husband, Doug Hampton, gain employment with a lobbying firm and his parents provided the Hamptons with a payment of nearly $100,000 that they described as a gift.
Watchdog groups have asked the Justice Department to investigate the payment as well as Hampton's interactions with his former boss and staff once he began lobbying for clients.
A person familiar with the FBI's review said agents recently contacted Ensign's former co-chief of staff, John Lopez. The person spoke on condition of anonymity because of the FBI's involvement and said the FBI did not issue a subpoena but only contacted Lopez. The person would not provide any other details about the FBI's discussions with Lopez.
In an ABC interview in late November, Hampton said no one from the FBI or Justice Department had contacted him. He discussed details about the affair and asserted his belief that ethics laws were broken. Hampton's attorney declined to comment Tuesday whether the FBI has contacted Hampton since the interview in November.
The FBI in Las Vegas declined to comment.
Ensign is not up for re-election until 2012. He repeated Tuesday that he has done nothing illegal and promised to cooperate with any investigation.