Chattanooga Terror Investigation Focuses on Suspect's Motive

— -- A full scale terrorism investigation using the FBI, local law enforcement, intelligence agencies and foreign allies is underway, trying to determine the motive behind Mohammad Youssef Abdulazeez’s attack on two military installations Thursday.

Senior government sources tell ABC News that the FBI and other agencies will first interview family, friends and associates to assess Abdulazeez’s state of mind and any shifts in personality and behavior that may provide clues to why he wanted to kill members of the U.S. military.

They also are going to focus on these areas to ascertain motive:

—Abdulazeez’s overseas travel: Officials want to trace his travel and see whether he secretly went to other countries beyond Jordan. Authorities are asking foreign intelligence services whether Abdulazeez ever came up on their radar and whether there is any indication he ever met with or had interactions with ISIS or other terror groups.

—Abdulazeez’s computer and phone: The FBI will conduct forensic exams on Abdulazeez’s phones and computers. Those items were sent to the FBI lab in Quantico, Virginia today.

—Abdulazeez’s presence on social media: There will be an intense focus on Abdulazeez’s social media footprint and whether the suspect had been using aliases online in an attempt to mask his activity. FBI Director James Comey has said the ISIS social media campaign has been successful in recruiting here in the U.S.

“Americans all over the place responding to this constant push and feed and buzz. It’s the devil on your shoulder all day long saying kill, kill, kill,” Comey said.

Authorities will look for evidence that Abulazeez may have used encrypted communication to hide potential contact with radicals. This is a recent tactic used by ISIS that Comey expressed concern about last week.

As of now, no clear motive has been established for the mass shooting, although many senior U.S. officials are deeply suspicious of an ISIS connection, because Abdulazeez did precisely what ISIS has been calling on followers to do — attack the U.S. military and law enforcement. But FBI Special Agent Ed Reinhold said today, “At this time we have no indication that he was directed or inspired by anyone other than himself.”

FBI sources say they are concerned that they are looking at a “nightmare scenario,” a suspect who was not on their radar — not under investigation, not under surveillance — who shows up out of nowhere and goes on a killing spree. This comes as the FBI wrestles with trying to track hundreds of suspected ISIS sympathizers across all 50 states.

The Department of Homeland Security and the military are taking steps today to shore up security at federal buildings and recruiting centers across the nation, in an effort to prevent any "copycat" attacks.