He was a naval defense contractor, and on Aug. 7, police alerted officials at the Newport Naval Station about his call. But officers didn't hear from him again.
By Aug. 25, Alexis had arrived in the Washington area, continuing his work as an information technology employee for a defense-related computer company. He suffered from serious mental problems and was undergoing treatment from the federal Department of Veterans Affairs, according to the law enforcement officials.
But Alexis wasn't stripped of his security clearance, and he kept working.
On Saturday, he visited a gun range in Virginia, not far from the nation's capital. He rented a rifle, bought bullets and took target practice, then bought a shotgun and 24 shells, according to the store's attorney.
On Monday, Alexis entered the sprawling Washington Navy Yard, which is protected by armed guards and metal detectors where employees must show IDs to get past doors and gates.
He had his pass for base access and the shotgun, a Remington 870 Express. The shotgun was brought into the building disassembled and pieced together by Alexis once inside, according to a law enforcement official and a senior defense official who spoke on the condition of anonymity because they were not authorized to discuss the matter publicly.
He opened fire around 8:15 a.m., shooting down from a fourth-floor overlook and third-floor hallway into a glass-walled cafeteria.
Alexis also took a handgun from an officer. He fired at police and workers in gun battles that lasted more than half an hour. The FBI says the average mass shooting is over within minutes and often ends once police arrive. Alexis was killed in a shootout.
The Navy said the military installation would reopen for business for Mission Essential personnel only Wednesday. In a posting on its Facebook page, the Navy said the yard remains an active crime scene.
Meanwhile, Defense Secretary Chuck Hagel ordered the Pentagon to review the physical security of all U.S. defense facilities worldwide and the security clearances that allow access to them. "Where there are gaps, we will close them," he said.
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Associated Press writers David Klepper in Newport, Rhode Island, and Brett Zongker in Washington contributed to this report.