A day after the assault in the nation's capital, the motive was still a mystery. U.S. law enforcement officials told The Associated Press that investigators had found no manifesto or other writings suggesting a political or religious motive.
They said Alexis, a 34-year-old information technology employee with a defense contractor, used a valid pass to get into the heavily secured Washington Navy Yard and then killed 12 people before he was gunned down by police in a shootout that last more than a half-hour.
Alexis had been undergoing mental health treatment from Veterans Affairs since August but was not stripped of his security clearance, according to the law enforcement officials, who spoke on condition of anonymity because the criminal investigation was still going on.
He had been suffering a host of serious mental problems, including paranoia and a sleep disorder, and had been hearing voices in his head, the officials said.
The assault is raising more questions about the adequacy of the background checks done on contract employees and others who are issued security clearances — an issue that came up most recently with National Security Agency leaker Edward Snowden, an IT employee with a government contractor.
Navy Secretary Ray Mabus ordered two security reviews Tuesday of how well the Navy protects its bases and how accurately it screens its workers.
A senior defense official also said Defense Secretary Chuck Hagel intends to order a review of physical security at all department installations worldwide.
Also, the House and Senates Veterans' Affairs Committees have asked the VA for details about any treatment provided to Alexis.
In the Newport, R.I., incident, Alexis told police he got into an argument with someone as he was getting on a flight from Virginia to Rhode Island, where he was working as a naval contractor, and he said that person sent three people to follow him and harass him.
He said he heard voices talking to him through a wall while at one hotel, so he changed hotels twice, but the voices followed him, according to the report. He said he feared they might harm him.
He also "stated the individuals are using a microwave machine sending vibrations to his body so he cannot fall asleep."
Later that day, Newport police alerted the Rhode Island naval station and a copy of the police report, Newport police Lt. William Fitzgerald said Thursday.
A spokeswoman for the station referred calls to the FBI.
During Monday's attack, Alexis was armed with a shotgun that he bought in Virginia, the FBI said. Officials said he also took a handgun from a law officer.
Alexis had run-ins with the law over shootings in 2004 and 2010 in Texas and Seattle, and his bouts of insubordination, disorderly conduct and being absent from work without authorization prompted the Navy to grant him an early — but honorable — discharge in 2011 after nearly four years as a full-time reservist, authorities said.
During his service, he repaired aircraft electrical systems at Fort Worth.