The Pentagon said Tuesday it would review security at US military bases worldwide one day after a defense contractor gunned down 12 people at the Washington Navy Yard, as investigators worked to uncover the shooter's motive.

Officials were struggling to answer how and why 34-year-old Aaron Alexis, a former sailor with a history of disciplinary problems and brushes with the law, had been granted a security badge before he opened fire Monday at the tightly guarded naval compound.

With lawmakers questioning the vetting of contractors, Defense Secretary Chuck Hagel "intends to order a review of physical security and access at all DoD (Department of Defense) installations worldwide," a senior Pentagon official said.

Federal authorities acknowledged there were many unanswered questions about Monday's mass shooting -- which sparked demands from Democratic lawmakers for a renewed debate on US gun laws.

"As we have moved from a crisis stage to the investigation stage of this case, our focus and efforts are going to be on the questions we all have," said US federal prosecutor Ronald Machen.

"What caused this individual to kill so many innocent men and women? How did he carry out and plan this attack? How did he get access to the weapons?"

Alexis, a former naval reservist hired as an IT subcontractor for computer giant Hewlett-Packard, had a valid pass to enter the Navy Yard, the Federal Bureau of Investigation said.

He entered the base Monday with a legally purchased shotgun, officials said, dispelling earlier reports that he had been armed with an assault rifle.

Alexis arrived in the Washington area on or about August 25th and stayed in local hotels, but his motive remained a mystery, said Valerie Parlave, assistant director of the FBI's Washington field office.

"We continue to conduct interviews, exploit digital media, and run down every lead we can to piece together his recent movements and determine the motive behind his attack," she said.

The New York Times reported that Alexis suffered from delusions, recounting how he called police in Rhode Island last month complaining about hearing voices and that people were sending "vibrations to his body" to prevent him from sleeping.

When police came to his hotel room on August 7 at 6:00 am, Alexis told them that he had gotten into an argument with someone at a Virginia airport, according to the Times, citing a Rhode Island police report.

Alexis reportedly said the person he had argued with "had sent three people to follow him and to keep him awake by talking to him and sending vibrations to his body" via a microwave machine.

Pentagon officials could not confirm if Alexis suffered mental health problems. He served in the Navy from 2007 to 2011, and officers said his record was marked by numerous cases of misconduct.

They ranged from minor offenses such as being late for work to more serious incidents involving disorderly conduct and insubordination, Navy officials said on condition of anonymity.

Alexis received non-judicial punishment by the Navy in some cases and he was arrested by civilian police at least twice -- once in Georgia for disorderly conduct and once in Texas when he fired a bullet through the ceiling of his apartment, they said.

The Navy sought to discharge Alexis for his behavior but had insufficient evidence of misconduct so they granted him an honorable discharge, the officials said.

It was not clear if Alexis was working at the Navy Yard at the time of the shooting, but the fact that he had a valid pass raised fresh questions about how the government vets its huge contractor workforce.

Hours after the shooting, the Pentagon's inspector general sent a report to Congress that alleged the Navy had eased security controls on outside contractors at many bases in an attempt to save costs.

Lawmakers expressed alarm at the audit's findings.

"We obviously have to review the procedures for the granting of security clearances," said Republican Senator John McCain.

Pentagon officials said the defense secretary was expected to order a review of the inspector general's report.

And the White House said Tuesday that President Barack Obama had ordered a review into security standards for contractors at federal agencies.