The FBI says it's too early to tell whether Mohammod Youssuf Abdulazeez was radicalized before he attacked two military facilities in Chattanooga, Tenn., and killed five service members.
"This is a complex, ongoing investigation, and we're still in the early stages of piecing together exactly what happened and why," FBI Special Agent in Charge Ed Reinhold said Wednesday at a news conference.
Reinhold said between 700 and 1,000 agents are working full time on the investigation into what led the Kuwaiti-born Abdulazeez, 24, to carry out last week's attacks.
"The FBI has been working almost 400 leads, and has an estimated 250 personnel on the ground in the area, and hundreds more working across the country and around the world on this investigation," Reinhold said.
He added the FBI was treating Abdulazeez, a naturalized American citizen, as a "homegrown violent extremist" at this point.
"We believe he acted on his own that day. We believe he entered the facility on his own," he said. "We do not have any indication that anyone else was assisting him on that day."
Abdulazeez fired at a military recruitment center and then drove to a naval reserve facility, where he killed four Marines and a Navy petty officer.
Three of the weapons recovered at the naval facility belonged to the shooter, Reinhold said; one was found in his rented Mustang, and two were found on him after his death. All the service members who were killed were hit by the same weapon, he said.
Reinhold said the FBI knew where the weapons were purchased, though he declined to share that information with the media.
Three other weapons were recovered at the scene, Reinhold said, and they belonged to service members. One of those weapons was fired at Abdulazeez, Reinhold said, though it's still unclear whether that's what killed the shooter.
Speaking at the same news conference, Maj. Gen. Paul W. Brier, commanding general of the 4th Marine Division, said the Marines responded to the attack as he would have expected.
"Rapidly going from room to room, they got their other Marines to safety," he said. "Once they got them to safety, some willingly ran back to the fight. All of us can be extremely proud of what our Marines did that day."
Abdulazeez's family says he suffered from depression, an assertion the FBI said it is investigating.