WASHINGTON, D.C. (CBS) - Washington Navy Yard gunman Aaron Alexis tried to buy an AR-15 assault rifle at a Virginia gun store last week after test firing one, but the store wouldn't sell it to him.
The reason for the refusal isn't clear.
Alexis then purchased a shotgun he used in his rampage, sources say.
Twelve people died before Alexis was shot and killed by police.
While law enforcement officials haven't pinned down a motive for the massacre, Alexis was clearly a troubled man with a history of angry outbursts.
His relatives have told investigators Alexis struggled with mental problems for at least ten years, and recently sought treatment from the Department of Veterans Affairs.
Police records reveal a troubling incident that occurred six weeks ago.
Alexis called Newport, R.I. police, claiming he was "hearing voices."
The defense contractor also complained that people were stalking him and using some kind of microwave machine to send "vibrations into his body."
But, in the police report, Alexis denied ever having a mental health concern.
Newport police say they told the Navy about the incident because Alexis said he was a Navy contractor. It was unclear whether the Navy followed up.
Until recently, Alexis had lived with Melinda Downs in her Texas home. She said he was sometimes withdrawn, but never violent.
While investigators still don't know what triggered Monday's massacre, they are getting a better picture of how it happened.
Sources say Alexis drove a rented Toyota Prius to the Navy Yard and used a valid pass to gain entry. He was carrying a bag police believe contained the shotgun used in the attack. He walked directly to Building 197, assembled his weapon and began shooting.
Sources say Alexis went to a fourth floor balcony and opened fire on the people in an open atrium below. He repeatedly moved up and down stair and at one point, killed a security guard and stole his pistol.
With the extra weapon, he began moving through the hallways.
There, some 30 minutes after the shooting had started, Alexis was confronted by police, shot and killed.
Law enforcement officials retracing Alexis' recent movements say he had been in the Washington, D.C. area for the past three weeks.