Aaron Alexis, the man police say shot and killed 12 people in a Washington Navy Yard, reportedly called police to complain about people following him and that he was hearing voices. He sought mental health treatment from a nearby VA hospital, officials said. NBC's Pete Williams reports.

Aaron Alexis, the Washington Navy Yard shooter, rented an AR-15 semi-automatic rifle and used it for target practice at a Northern Virginia gun range and store less than two days before his shooting spree, according to the lawyer for the store.

J. Michael Slocum, a lawyer for Sharpshooters Small Arms Range in Lorton, Va., said in an email to NBC News that on Sept. 14 Alexis also bought a Remington 870 shotgun and a "small amount of ammunition -- approximately 2 boxes- 24 shells." Alexis listed his residence as being in Texas.

Authorities said Alexis used a Remington shotgun to shoot a police officer and a security guard before taking one of their handguns and continuing on his rampage. Some earlier reports said an assault-style rifle, such as the AR-15 was used in the shooting but the FBI said it didn’t believe he did.

The 12 men and women gunned down in the Washington, D.C., Navy Yard came from all walks of life, and had their lives stolen in one senseless moment. NBC's Tom Costello reports.

Slocum said he had no information on whether Alexis sought to purchase an AR-15 the day he visited Sharpshooters.

"In accordance with Federal law, Mr. Alexis' name and other applicable information, including his state of residency, was provided to the Federal NICS system and he was approved by that system," Slocum said in the email. "After the terrible and tragic events at the Navy Yard, Sharpshooters was visited by federal law enforcement authorities, who reviewed the Range's records, including video and other materials."

In the weeks before the attack on the Navy Yard, Alexis reported hearing voices and said that three people were sending vibrations through the ceiling to keep him from sleeping, police said Tuesday.

Police in Newport, R.I., said that Alexis called them to a Marriott hotel there on the morning of Aug. 7 and reported that he was being followed and was worried that the people were going to hurt him.

Alexis told police that the three talked to him through the walls, floor or ceiling at three hotels — two commercial hotels in Rhode Island and one on a naval base there. He told them that they used a microwave machine to send vibrations and keep him awake.

One day after the rampage, which left 12 people dead at the Navy Yard before officers gunned down Alexis, the FBI said that it was still looking for an explanation. An FBI official would not answer a reporter’s question about whether Alexis was simply mentally ill, with no deeper motive.

Asked about new gun-control measures in the wake of what seem to be steady stream of horrific shooting sprees, President Barack Obama lamented Congress' inability to take action.

“The fact that we do not have a firm enough background-check system is something that makes us more vulnerable to these kinds of mass shootings,” Obama said in an interview with Telemundo, adding that “ultimately this is something that Congress is going to have to act on."

Obama also ordered a review of government contractor and employee security across federal agencies.

Slideshow: Navy Yard shooting / A gunman opened fire Monday at the Washington Navy Yard in Washington D.C., killing at least 12 people, authorities said. Launch slideshow

As investigators looked for answers, Alexis’ profile became fuller. A friend from Texas said that Alexis was so unhappy with his life that he was ready to leave the country, and that he was beset by money problems and felt slighted as a veteran. Alexis was seen by Veterans Affairs twice, including recently in Rhode Island, law enforcement officials said.

Federal law enforcement officials sketched an early account of the spree to NBC News. They said that Alexis left a Residence Inn in Washington on Monday morning, drove a rental car to the Navy Yard and used his credentials to enter the base.

Military officials said that Alexis, 34, a former Navy reservist who was working as a civilian contractor, had a security card that allowed him access to the Navy Yard but not to the office building where he later opened fire.

Once on the base, the law enforcement officials said, Alexis apparently entered a men’s room and readied a Remington pump-action shotgun. They said he walked out, shot a police officer and a security guard and took at least one of their handguns before continuing his rampage.

Inside the headquarters of the Naval Sea Systems Command, on the fourth floor, Alexis sprayed bullets into a breakfast crowd in an atrium below, at others on a third-floor balcony and at at least one man in a fourth-floor corridor, according to authorities and eyewitness accounts.

A senior investigator told NBC News that authorities believed Alexis used the handguns, which were semi-automatics, after emptying the Remington into his victims. Evidence technicians were still canvassing the crime scene late Tuesday and had not finished counting the rounds and shotgun pellets.

Alexis bought the shotgun legally in Virginia, said Chief Cathy Lanier of the Washington police.

Valerie Parlave, the assistant director in charge of the Washington FBI field office, said that agents did not believe Alexis used an AR-15 military-style assault rifle. Some law enforcement officials had said Monday that investigators believed he used such a weapon.

Parlave said that the spree was conducted with a shotgun and perhaps a handgun.

"I do get concerned that this becomes a ritual that we go through every three, four months, where we have these horrific mass shootings," Obama told a Telemundo reporter on Tuesday evening. "Everybody expresses understandable horror. We all embrace the families and our thoughts and prayers are with those families right now as they're absorbing this incredible loss. A yet we're not willing to take some basic actions."

Alexis was killed after exchanging gunfire repeatedly with officers who swarmed the Sea Systems Command as part of a massive law enforcement response.

Authorities have named all 12 of the people slain in the shooting. Two people remain hospitalized with wounds and are expected to recover. One woman who had wounds to her hands and head was released Tuesday evening. Five other people were treated for medical issues related to the rampage and released.

Alexis appears to have had a history of psychological problems and was deteriorating, and military officials said he had a disciplinary record that included insubordination and disorderly conduct.

Further in his past, Alexis was accused three years ago of accidentally firing a bullet into an upstairs apartment, and nine years ago of shooting the tires of another person’s car during an anger-fueled blackout in Seattle, where he had family. At that time, his father told detectives that Alexis helped with rescues after the Sept. 11 attacks and suffered post-traumatic stress.

White House spokesman Jay Carney said Tuesday that Obama has ordered his Office of Management and Budget to examine security standards for contractors and employees across federal agencies

NBC's Tom Costello spoke with the heroic U.S. Park Police officers who flew their helicopter into the 'hot zone' during the active shooting scene at Navy Yard. Their chopper hovered as the scene unfolded, providing logistical support and rescuing a shooting victim.

Navy Secretary Ray Mabus said he plans to order two reviews to examine security procedures at all Navy and Marine Corps installations, a senior defense official told NBC News.

The first will be a “quick look” at physical security requirements of bases, the official said. The second will be a deeper look at both the physical and personal security requirement.

The personal requirements include whether someone is likely to protect classified information and adhere to standard security procedures. The physical review is a deeper look at physical security requirements on a base: swipe access, perimeter security, patrols, etc.

The official could not provide a timeline for the reviews.

Mabus’ reviews come as a Defense Department inspector general report, made public on Tuesday, disclosed major flaws in security screening of contractors working on Navy installations.

The report says 52 convicted felons were able to routinely get on bases even though their felony convictions came before they were issued entry credentials. “Numerous” contract employees received such credentials without proper vetting through authoritative databases such as the National Crime Information Center and the Terrorist Screening Databases, the report found. Plus, contractors were too easily allowed to get local day passes without the mandatory screening.

"While there may not be a direct link between the result of this report and the horrific loss of life, I am deeply concerned about the current security situation at Navy facilities,” U.S. Rep. Mike Turner, R-Ohio, wrote to Mabus Tuesday after a briefing by the inspector general. "It is unacceptable that the Navy has granted installation access to individuals without performing thorough and complete background checks." He called for "immediate action to remedy all security gaps.”

Authorities say that Alexis is believed to be the only person responsible for the rampage. For a time Monday, Washington police said they were looking for as many as two people who were possible accomplices.

Alexis was working for a subcontractor for Hewlett-Packard Enterprises called The Experts, which does work in the Navy Yard complex. A spokesman for the company said Tuesday that Alexis had passed two background checks, most recently in June, and that the company confirmed twice through the Defense Department that Alexis had security clearance.

The June background check found no problems beyond a traffic violation, the spokesman said.

NBC's Justice Correspondent Pete Williams joins NewsNation's Tamron Hall to report on the emerging picture of mental health issues in the man who shot and killed 12 people at the Washington Navy Yard.

Alexis joined the Navy Reserves in May 2007 and was discharged in January 2011. The discharge was honorable and was granted early in what is known as an enlisted transition.

Military officials acknowledged, however, that Alexis had disciplinary problems including absence without permission, insubordination and disorderly conduct.

The officials said that often in cases like this the individual is offered early voluntary discharge instead of dishonorable discharge or a general discharge, which is one step below honorable. The officials could not say that that was the case with Alexis.

Defense Secretary Chuck Hagel laid a wreath Tuesday at the U.S. Navy Memorial in Washington, and a bugler played taps. Washington police released the names of the dead as their families were notified. Authorities said none of the victims was known to have been active-duty military personnel.

The spree was the deadliest U.S. shooting since the Sandy Hook Elementary School massacre last December, the deadliest on a U.S. military installation since 13 people were killed at Fort Hood, Texas, in 2009.

It was also the single deadliest incident in Washington since Sept. 11, 2001.

Jonathan Dienst, Erin McClaim, Jason Cumming, Richard Esposito, Charles Hadlock, Jeff Black and Andrew Rafferty of NBC News contributed to this report.

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