WASHINGTON — Six weeks ago, Aaron Alexis told people someone had threatened him at an airport in Virginia. A few days later, in Rhode Island, he heard voices. He thought people were speaking to him through "the walls, floor and ceiling" of the Navy base there, where he was working.

In his hotel room, the voices used "some sort of microwave machine" to send vibrations through the ceiling and into his body, a police report shows him saying. He could not sleep.

This article wrongly provides a full name in place of an acronym in a quotation from the lawyer for the Sharpshooters Small Arms Range and gun store in Lorton, Va. Attorney J. Michael Slocum said the store had provided Aaron Alexis’ information to the NICS, the National Instant Criminal Background Check System, not the federal Naval Criminal Investigative Service.

Alexis frequently moved as part of his contract work at military installations from New England to North Carolina; he arrived in Washington on Aug. 25. He switched hotels several times until Sept. 7, when he finally settled into the Residence Inn — a mile from his new workplace at the historic Washington Navy Yard on the capital's waterfront.

On Saturday he visited a gun shop in the Virginia suburbs. He practiced firing a rifle, then purchased a Remington 870 shotgun and 24 shells. The short-barrel weapon, known popularly as a "riot gun," is commonly used by police and the military.

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On Monday he reported to work with that shotgun. The FBI says he had a valid pass to enter the base. At 8:15 a.m., in Building 197, the most crowded structure there, he opened fire, grabbed a pistol along the way, and killed 12 people, shooting at police until they killed him in a gun battle that lasted about half an hour.

A day later, Alexis' history of mental problems, his extensive disciplinary record from his time in the Navy, and his three arrests over the last decade — two of them for gun-related incidents — have generated numerous questions.

Many are reminiscent of past mass shootings: How had police, the military and the company he worked for missed the accumulating signs of trouble? Why was the 34-year-old loner and drifter given an ID card that would allow him to easily come and go from military bases around the country without a security check? How could he so readily pass a background check to buy a shotgun?

Amid those questions, Defense Secretary Chuck Hagel plans to order a review of security procedures at all Defense Department installations in the U.S., a Pentagon official said Tuesday.

At the company Alexis worked for, the Experts, an information technology firm based in Ft. Lauderdale, Fla., Chief Executive Thomas E. Hoshko said that despite having paid another company to conduct a background check on Alexis before hiring him in 2012, to Hoshko's knowledge his company was "never made aware of any criminal or health issues."

"I have more questions than you, and I am working to find out what can be done to improve security on bases, as well as the security process," he wrote in an email.

Alexis' secret-level security clearance, which he originally received in 2008 after joining the Navy, made him a valuable hire for an IT company with contracts to work on classified computer networks. And it allowed him entry to the Navy Yard without being searched.

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Like most military personnel, Alexis got his clearance as a routine matter so he could access the computers that he might use on a daily basis in his job as an electronics expert on C-40 cargo planes at Fort Worth Naval Air Station, Navy officials said.

The clearance was good for a decade, officials said. Although his Navy record included several unauthorized absences from duty, instances of insubordination and disorderly conduct, one case of being absent without leave and several failed inspections, none of the problems rose to a level that would have jeopardized his clearance, they said.

When Alexis was discharged in 2011, his clearance became inactive, but it was reinstated without the need for additional investigation when he went to work for a contractor, officials said.

"The security clearance system is not foolproof," said Steven Aftergood, a secrecy and security expert with the Federation of American Scientists. "But what is reasonable to expect is that evidence of past criminal activity and a propensity to violence should be detected, and in this case the process failed to do that."