FBI Director James Comey said Aaron Alexis appeared to be "hunting people" with "all different backgrounds."
Comey offered a detailed account of the shooting. He said Alexis drove a rental vehicle to a parking deck across from the Navy Yard's Building 197 shortly before 8 a.m. Carring a duffel bag, he then cross the street and entered the building using a valid pass. He had a Remington 870 pump-action shotgun inside the duffel bag, Comey said.
Alexis went up to the fourth floor and entered a men's restroom. After he left the restroom, he began shooting people in "no discernable pattern," Comey said. He then went down to the first floor and opened fire on a security guard, and then taking his Beretta handgun. Alexis proceeded back to the third and fourth floors, gunning down more people in the hallway and offices.
According to Comey, the first armed responders didn't engage Alexis until 8:30 a.m.
The FBI director has acknowledged Alexis' deteriorating mental health. ABC News has learned that on the stock of his shotgun, Alexis had etched the phrases "better off this way" and "my elf weapon."
Three days after the mass shooting, normal operations resumed Thursday at the Navy Yard, and some workers returned to work. But it was anything but normal.
"I'd rather not be here today," said Judy Farmer, a scheduler from Manassas, Va.
Bob Flynn, who hid in an office in Building 197 with four colleagues during the shooting, said it helped to be at work with them.
"I feel good because I got to see my co-workers that I went through this with," Flynn said. "I get to hug people, and everybody gets the hugs and we get to talk about it and I think it's going to be helpful."
Secretary of the Navy Ray Mabus met with the workers in the morning, according to Flynn.
"He said, 'If anybody has a problem, you call me,' and he means it," Flynn said. "It's just one big family, and that's why we're going to be able to make it."
ABC News and The Associated Press contributed to this report.