Dead is Alexis, a former sailor with a
"pattern of misconduct," and 12 others, a mix of civilian workers and military contractors.
Despite earlier reports suggesting as many as two other people may have been involved, Washington police are confident that only one person was involved in Monday morning's shooting at the Washington Navy Yard, police Chief Cathy Lanier said Monday night.
What motivated Alexis to kill was a mystery Tuesday
Terrorism hasn't been ruled out but seems unlikely, Gray told reporters.
A friend and former housemate, Kristi Suthamtewakul, told CNN's "New Day" that she had noticed personality changes in Alexis over the last few months, but nothing indicating the potential for such violence.
"Aaron was a very polite, very friendly man," she said.
Among other problems, he had been frustrated about pay and benefits issues after a one-month contracting stint in Japan last year, Suthamtewakul said.
"He got back and he felt very slighted about his benefits at the time," she said. "Financial issues.
He wasn't getting paid on time, he wasn't getting paid what he was supposed to be getting paid."
"That's when I first started hearing statements about how he wanted to move out of America," Suthamtewakul said. "He was very frustrated with the government and
how as a veteran he didn't feel like he was getting treated right or fairly."
Another friend, Texas resident Michael Ritrovato, said Alexis had recently been
frustrated with his employer over pay.
But Ritrovato said his friend never showed signs of aggressiveness or violence,
though he played a lot of shooting video games online
"It's incredible that this is all happening, because he was a very good-natured guy," Ritrovato said. "It seemed like he wanted to get more out of life."
In Seattle, police said they arrested Alexis in 2004 for shooting out the tires of another man's vehicle in what Alexis later told detectives was an anger-fueled "blackout."
He had a "pattern of misconduct" while in the Navy, but was honorably discharged, two Navy officials told CNN.
Authorities have recovered three weapons from the scene of the mass shooting, including a shotgun that investigators believe Alexis brought into the compound, federal law enforcement sources with detailed knowledge of the investigation told CNN on Tuesday. The other two weapons -- handguns -- may have been taken from guards, the sources say.
Earlier information that Alexis may have used a semiautomatic AR-15 rifle may have been incorrect, the sources said.
Investigators believe Alexis rented an AR-15 but had returned it before Monday's shootings, the sources said.