What we know: Washington Police Chief Cathy Lanier said Tuesday that Alexis had "multiple contacts" with officers from several agencies during the shooting spree. The ordeal ended when Washington police Officer Scott Williams fatally shot Alexis, 34, Mayor Vincent Gray told CNN.
What we don't know: Exactly how Alexis died, beyond the fact it happened after 30 minutes of gunfire, according to Gray.
What we know: He left New York after apparently helping rescue efforts at the site of the World Trade Center after the September 11, 2001, terror attacks, according according to a source with direct knowledge of the investigation. His father told Seattle police after a 2004 arrest that Alexis was suffering from post-traumatic stress disorder related to that experience.
He had served in the Navy, but officials moved to discharge him in 2010 over a pattern of misconduct that included insubordination, unauthorized absences and other infractions, according to a U.S. defense official. It wasn't enough for a general discharge, so Navy officials decided to grant him an honorable discharge, the official said.
As a contractor, he worked in information technology, had medium security clearance, high enough to work at multiple Navy offices over the summer. He had an ID badge to enter the Navy Yard. His employer says the shooter jumped through all the right hoops. "Alexis had a security clearance that was updated in July, approved by military security service personnel," said Thomas Hoshko, CEO at The Experts. "There is nothing that came up in all the searches."
He appeared to become increasingly troubled in recent years, according to the source with direct knowledge of the investigation. He was hearing voices and having problems sleeping, the source said. He was growing increasingly troubled and in recent months had exhibited signs of mental problems, according to the source. Based on family accounts, it appears Alexis "basically snapped," the source said.
On August 7, police in Newport, Rhode Island, investigating a harassment complaint at a Marriott hotel encountered Alexis. He told them that an individual "had sent three people to follow him and to talk, keep him awake and send vibrations into his body," according to a police report. These individuals, Alexis police, had followed him to three hotels in the area -- including talking to him through walls and the floor and using "some sort of machine" to send vibrations through the ceiling that prevented him from falling asleep.
Police subsequently notified officials at a Navy base in Newport, the police report stated. Officials at Naval Station Newport referred CNN to the FBI on Tuesday when asked about the incident, and the FBI subsequently had no comment on the August police report.
What we don't know: How he was able to get a security clearance with such a spotty background.
Former Navy SEAL Cade Courtley says a poor or incomplete background check is to blame. "Most people, when they get into that, they are given an interim clearance, and that means that the background check hasn't been done but it's in the process of being done," Courtley said. "He may have started out with an interim clearance and a background check should have been done."
The former SEAL says just running Alexis' fingerprints would have turned up his arrest record. In Seattle, he fired several shots into the tires of a car during an altercation over construction near where he lived in 2004. There was also a weapons incident in Texas in 2010.
SECURITY AT THE YARD
What we know: Alexis drove onto the grounds of Navy Yard on Monday morning with a shotgun, according to federal law enforcement sources with detailed knowledge of the investigation. He proceeded into Building 197 and opened fire. He had access to the Navy Yard because of his contracting work, and he used a valid pass to gain entry.
City police were the first law enforcement personnel on site within eight minutes of when the first shots rang out, according to Gray, and they were joined soon by SWAT team members.
Washington Police Chief Cathy Lanier said there's "no doubt in mind" that law enforcement officers response to the shooting "saved numerous lives."
What we don't know: Even to drive or walk onto the base, a person would be required to present credentials, said Navy Capt. Mark Vandroff. Building 197 has armed security at the door. How did he get the guns past them? Did cost-cutting compromise Navy security? Rep. Mike Turner, an Ohio Republican and a member of the Armed Forces Committee, thinks so. He wants a congressional briefing from the Pentagon inspector general on a Navy security audit that he says was released after Monday's shooting.
"It is my understanding that the IG report indicates the Navy may have implemented an unproven system in order to cut costs," Turner said. "I also learned that potentially numerous felons may have been able to gain restricted access to several military installations across the country due to insufficient background checks, increasing the risk to our military personnel and civilian employees."