The FBI raided a home early Saturday next door to the house where one of the suspects in a deadly California shooting once lived with his family.
Armed authorities broke windows and used a cutting torch to get into the garage, neighbors said.
Syed Farook, 28, and his wife, Tashfeen Malik, 29, killed 14 people in a commando-style shooting rampage on Wednesday in San Bernardino.
They were killed in a furious shootout with police hours after they armed themselves with assault rifles and opened fire on a gathering of Farook's colleagues from the San Bernardino County health department, where he worked as a restaurant inspector.
The FBI said it is investigating the rampage as a 'terrorist attack'.
The FBI would not say what it was looking for during the raid on the home on Saturday, but a neighbor said a friend of Farook lives there.
More than three years ago, that person bought two assault rifles later used in the shooting, but authorities haven't been able to talk to him because he checked himself into a mental hospital after the attack, according to a law enforcement official who was not allowed to discuss the investigation and spoke on condition of anonymity.
The FBI has said the man is not a suspect in shootings, though they want to question him.