Washington - A month before he went on the rampage that left 13 dead, Washington Navy Yard gunman Aaron Alexis complained to police in Rhode Island that people were talking to him through the walls and ceilings of his hotel rooms and sending microwave vibrations into his body to deprive him of sleep.

The account, contained in an Aug. 7 report by Newport, R.I., police, adds to the picture that has emerged of an agitated and erratic figure whose behavior and mental state repeatedly had come to authorities' attention but didn't seem to affect his security clearance.

Alexis, a 34-year-old information technology employee at a defense-related computer company, used a valid pass Monday to gain access to the Navy Yard and kill 12 people before he was slain by police in a shootout that lasted more than a half-hour.

A day after the assault, 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 motivation.

Alexis, a former Navy reservist, had been undergoing Veterans Affairs mental health treatment since August but retained his security clearance, according to the law enforcement officials, who spoke on condition of anonymity because the criminal investigation was ongoing.

He had been suffering 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 who hold security clearances - an issue that came up recently with National Security Agency leaker Edward Snowden.

Navy Secretary Ray Mabus ordered two security reviews Tuesday of how well the Navy protects its bases and how accurately it screens its workers.

Similarly, President Barack Obama has ordered the White House budget office to examine security standards for government contractors and employees across federal agencies.

In addition, the House and Senate Veterans' Affairs Committees asked the VA for details about any treatment provided to Alexis.

Mourning the dead

At the U.S. Navy Memorial, in church and on the baseball field, the nation's capital paused to mourn the victims. Defense Secretary Chuck Hagel laid a wreath at the memorial's "Lone Sailor" statue as taps played.

Just a few blocks from the Navy Yard, the Washington Nationals were back to playing baseball after their Monday night game with the Atlanta Braves was postponed because of the shooting. The Nationals wore blue and gold Navy caps during warm-ups, and a moment of silence was held before the first pitch.

Those killed included: Michael Arnold, 59, a Navy veteran and avid pilot who was building a light airplane at home; Sylvia Frasier, 53, who worked in computer security; Frank Kohler, 50, a former Rotary Club president in Lexington Park, Md., who proudly reigned as "King Oyster" at the annual seafood festival; and marine engineer and naval architect Vishnu Pandit, 61, an Indian immigrant.

In the Newport incident, Alexis told police he had gotten into an argument with someone as he was getting onto a flight from Virginia to Rhode Island, where he was working as a naval contractor, and he said the person sent three people to follow 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 that the individuals are using 'some sort of microwave machine' to send vibrations through the ceiling, penetrating his body so he cannot fall asleep."

Later that day, Newport police alerted the Rhode Island naval station and staff there sent a copy of the police report, Newport police Lt. William Fitzgerald said Thursday.

A spokeswoman for the station had no comment Tuesday.

Bought gun Saturday

Alexis came to the Washington area about two weeks later and had been staying at hotels. On Saturday, two days before the attack, he went to a Virginia gun store about 15 miles from the Navy Yard.

He rented a rifle, bought bullets and took target practice at Sharpshooters Small Arms Range, the store's attorney Michael Slocum said. Alexis then bought a shotgun and 24 shells, according to Slocum.

The FBI said that during Monday's attack, Alexis was armed with a shotgun. Officials said he also took a handgun from a law officer.

Alexis had run-ins with the law in 2004 and 2010 in Texas and Seattle after he was accused of firing a gun in anger. He was not prosecuted in either case.

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.

Alexis joined the Florida-based IT consulting firm The Experts in September 2012, leaving a few months later to return to school. He came back in June to do part-time work at the Washington Navy Yard as a subcontractor, helping the military update computer systems.

The Experts' CEO, Thomas Hoshko, said that Alexis had "no personal issues," and he confirmed that Alexis had been granted a "secret" clearance by the Defense Security Service five years ago.

Alexis' clearance - lower than "top secret" - doesn't need to be renewed for 10 years. Still, the company said it hired outside vendors twice to check Alexis' criminal history.

Alexis' background check "came back clear," Hoshko said.

Associated Press writers Pauline Jelinek, Matt Apuzzo, Adam Goldman, Jesse Holland, Stacy A. Anderson, Brian Witte and Ben Nuckols in Washington contributed to this report.