FBI agents are at the home of the woman whose affair with retired General David Petraeus led to his resignation as CIA director.
A spokeswoman for the FBI said that agents went to Paula Broadwell's home in Charlotte, North Carolina. FBI spokeswoman Shelley Lynch declined to say what the agents were doing there.
FBI agents appeared at Ms Broadwell's home carrying the kinds of cardboard boxes often used for evidence gathering during a search.
They walked through the open garage of Ms Broadwell's house and knocked at a side door before entering the home.
One person was taking photographs of the house and its garage as members of the news media watched.
Gen Petraeus resigned his CIA post last Friday after acknowledging an extramarital affair.
The top American commander in Afghanistan, General John Allen, is also under investigation for alleged "inappropriate communications" with Jill Kelley, the woman who is said to have received threatening emails from Broadwell.
US defence secretary Leon Panetta said the FBI referred the matter to the Pentagon on Sunday. Mr Panetta says he ordered a Pentagon investigation of Gen Allen.
A senior defence official, who discussed the matter only on condition of anonymity because it is under investigation, said Gen Allen's communications were with Ms Kelley, who has been described as an unpaid social liaison at MacDill Air Force Base, Florida, which is headquarters to the US Central Command.