Aaron Alexis tried to buy assault rifle but was unable to

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 right away, CBS News has learned.

The reason for the refusal isn't clear.

Alexis then purchased a shotgun he used in his rampage, sources tell CBS News.

The owners of two gun stores in Virginia told CBS News Alexis would have been able to buy an AR-15, he just wouldn't have gotten it right away.

Anyone can buy the assault-style weapon in Virginia, but the dealer would have to observe the laws of the buyer's home state.

It appears unlikely Alexis was a Virginia resident. His last reported full-time residency was in Texas.

If the buyer is an out-of-state resident, the dealer would then ship the weapon to the buyer's home state where a background check would be conducted. At the time of purchase in Virginia, however, the buyer would have to show two proofs of residence with matching addresses and then a proof of citizenship. This is all according to federal law when it comes to sales of the AR-15, which are administered by the ATF.

It is also unclear whether the owner of Sharpshooters Small Arms Range in Lorton, Virginia, where Alexis tried to buy the AR-15 about 15 miles from the Navy Yard, told Alexis he was not allowed to buy an assault rifle, or whether Alexis was simply impatient and decided to buy a shotgun instead.

Twelve people died before Alexis was shot and killed by police.

CBS News correspondent Bob Orr reports that, 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.

CBS News correspondent John Miller reports a recent Inspector General report troubling findings when investigating security clearances at the nation's military bases.

At seven out of 10 bases, they found convicted felon getting common access cards.

Due to the fact that there's five million people with security clearances, the Department of Defense has been farming out background checks to private companies. They're not finding derogatory data when they do the check, because these companies are often using public, and therefore not thorough, databases to look into individuals' pasts.

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.