The house of the 24-year-old gunman, who was killed after shooting four Marines dead in Tennessee, is being searched, U.S. police said.
A district lawyer said that the attacks at two U.S. Navy sites on Thursday morning were being probed as an 'act of domestic terrorism', reported the BBC.
At this stage, the motive of the assailant, Mohammad Youssuf Abdulazeez, is not clear, the Federal Bureau of Investigation (FBI) said.
Three people were injured in the shooting, including a female sailor, who remains in a serious condition in hospital.
Police have cordoned off the area around the gunman's house.
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Eye-witnesses said that two women were taken away from the house in the Hixson suburb of Chattanooga in handcuffs.
FBI agent Ed Reinhold told the press that the gunman initially fired from his Ford Mustang at about 10:45 am at a US Navy recruitment centre in the east of the city and then drove to a U.S. Navy reserve centre, about seven miles away, where he got out of his car and shot the marines.
He was shot dead on Amnicola Highway.
U.S. President Barack Obama termed the attack 'heartbreaking.
