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The FBI is leading the investigation into Monday's deadly Navy Yard shooting rampage.
Several red flags have emerged, showing shooter Aaron Alexis, 34, had a history of troubling mental illness.
FBI agents investigating Alexis will talk to co-workers and friends, both past and present, to find out what could have led the 34-year-old Navy contractor to snap, said former FBI executive assistant director Shawn Henry.
"The FBI is looking for motive right now. That's one of their primary priorities. They want to know everything about this man that they can find out. They will go back to grade school if they have to," said Henry.
Indeed, a source told CNN's Jake Tapper the FBI has gone back ten years ago, to a place where Alexis was working in New York City.
"They will run these things to ground to make sure that at the end of the day they know exactly who he was, what he did, and what his motive was," said Henry.
Questions remain over how Alexis accessed the Navy Yard carrying, at the very least, a shotgun. According to law enforcement officials, Alexis had a valid pass to the base. Henry said guards likely do not search every single vehicle that passes through the base.
"I imagine that they would be doing that periodically, spot-checking, but if they were to do that and shut down this naval base, they have got 3,000 people that work here, that would be somewhat onerous to do that," said Henry.
"If he had a valid pass, they probably would have allowed him in," said Henry.