From there, he headed to a perch overlooking the building's atrium and began firing on those below using 00 buckshot shells, each packed with about a dozen pellets capable of causing tremendous damage, according to the same official.
Parlave said Alexis is believed to be solely responsible for Monday's bloodshed that, in addition to those killed, left eight wounded. Three of them suffered gunshot wounds, one of whom was released Tuesday. Another is Washington police Officer Scott Williams credited with killing Alexis. Still, that doesn't mean authorities aren't looking into others who might have helped Alexis, wittingly or unwittingly, or known something about the plot.
Federal investigators on Tuesday collected Alexis' computer and possessions from the hotel where he spent the last few days of his life, in addition to reviewing surveillance and other tools to see whom he interacted with in three weeks around Washington.
Co-workers have portrayed Alexis of having lived the mundane work life of a well-paid tech contractor given daily per diems that allowed a comfortable stay in an expensive city, a senior law enforcement official said.
Authorities are also appealing for the public's help in a probe U.S. Attorney Ronald Machen estimates could take "weeks and months."
"No piece of information is too small," Parlave said.
'Who was this guy?'
A New York City native -- where both his parents, now divorced, still live -- Alexis worked between 2001 and 2003 at the Borough of Manhattan Community College. His supervisor there, Barry Williams, told CNN he never would have expected such a violent outburst, though Alexis would get easily frustrated over minor things and hold grudges.
Years later, Alexis joined the Navy -- which admitted him knowing of his 2004 arrest, having been apparently "satisfied that it did not preclude granting (secret security) clearance," a senior Navy officer said -- as a petty officer working on electrical systems.
But his four years in service weren't all smooth. He was written up for eight instances of misconduct on duty, a U.S. defense official told CNN, including cases of insubordination, disorderly conduct, unauthorized absences and at least one instance of intoxication.
"He wasn't a stellar sailor, we know that," said Rear Adm. Kirby, adding that the misconduct cases were all "relatively minor." "... None of those (offenses) give you an indication he was capable of this sort of brutal, vicious violence."
Rep. Michael McCaul, chairman of the House Homeland Security Committee, said he feels the infractions "were kind of swept under the rug."
"It is real easy to just pass the buck along to another military base or in this instance, a defense contractor," the Texas Republican said Tuesday. "...There are so many red flags that popped up in this case."
Without a civilian conviction, the offenses weren't enough to produce a general discharge; Alexis was granted an honorable discharge in January 2011 instead. In fact, he remained part of the Navy's Ready Reserve up to his death, Navy Secretary Ray Mabus told CNN.
The Experts -- the contracting firm for which Alexis worked for about six months over the past year -- said the last of two background checks it conducted in June on Alexis "revealed no issues other than one minor traffic violation."
Still, did something change more recently? There was the August incident in Rhode Island. And a friend and former housemate, Kristi Suthamtewakul, told CNN's "New Day" she 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."