New revelations about the Navy contractor who killed 12 people at a secure military installation here have exposed serious shortfalls in security screening and base protection, prompting the White House and Pentagon to order a global review.

President Barack Obama and Defense Secretary Chuck Hagel called for a broad security assessment as the administration and law-enforcement officials tried to determine how a Navy reservist with a yearslong history of mental-health problems and run-ins with the law was able to carry out the deadly attack.

Officials investigating Monday's shooting painted a portrait of a troubled assailant who said he was hearing voices and believed as recently as last month that adversaries were using a 'microwave machine' to prevent him from getting sleep.

Aaron Alexis, the 34-year-old former Navy reservist killed by officers responding to the attack, had sought treatment earlier this year from the Department of Veterans Affairs for paranoia and other complaints, according to officials familiar with the investigation.

What appeared to be growing mental instability failed to prompt officials to strip Mr. Alexis of his security clearance and access to the Washington Navy Yard, where he went on his shooting spree

Monday's shooting cast a spotlight on the Navy's policies for granting contractors access to military bases, a problem highlighted in an unconnected report released Tuesday by the Defense Department Inspector General.

The security lapses raised new concerns for lawmakers from both parties.

'There was no doubt that this was somebody who had a record of instability and certainly should have been, I think, the subject of closer scrutiny, particularly in access to a facility such as the Navy Yard,' said House Minority Whip Steny Hoyer (D., Md.).

Mr. Alexis, who worked for a computer technology firm, was one of more than one million private contractors to hold 'confidential' or 'secret' government security clearance. That type of clearance requires a lower level of scrutiny than the 1.4 million people with top government security clearance.

Navy officials said Mr. Alexis was first granted security clearance in 2008, soon after he joined the reserve. He retained it after he was honorably discharged from the military in January 2011. Superiors had considered pursuing him a less-than-honorable discharge.

While Mr. Alexis had a series of run-ins with police involving use of firearms, none led to convictions that would have triggered a revocation of his security clearance.

And, though he had a spotty record in the Navy, the discipline issues were relatively minor, including insubordination, messy quarters and being late to his post. A Navy official said security clearances are typically revoked for adultery, gambling, alcohol abuse and other infractions that could be used against a sailor.

'His misconduct did not rise to a level his security clearance would be revoked,' said a Navy official.

Rear Adm. John Kirby, the chief of information for the Navy, said the Navy would carry out an in-depth review of Mr. Alexis's service and his security clearance. 'We are willing to acknowledge it if there are missed flags, and we are taking a look right now,' he said.

James Lewis, a director of the Technology and Public Policy Program at Center for Strategic and International Studies, said that to be denied clearance, 'you basically have to have committed a crime, been hospitalized [for mental problems] or have something in your background that would suggest a tie to a foreign power. Those are the red lines. The question now is do we need to screen for more?'

Concerns about Mr. Alexis come four months after another military contractor, Edward Snowden, used his top-secret clearance to obtain scores of classified documents about U.S. surveillance programs and release them to journalists around the world. Mr. Snowden's ongoing revelations about the National Security Agency triggered a review of the security-clearance process and calls from lawmakers to tighten controls on government contractors.

As lawmakers pressed for hearings on Mr. Alexis, the Pentagon Inspector General released a report Tuesday that said the Navy had granted access to some of its bases -- including the Navy Yard -- without proper background checks.

The report concluded that lax controls allowed 52 people convicted of felonies -- including assault, theft and 'indecent liberties with a child' -- to routinely get on and off Navy bases, 'placing military personnel, dependents, civilians and installations at an increased security risk.'

Navy officials said while they are reviewing the report, the system examined by the inspector general -- the Navy Commercial Access Control System -- wasn't used by Mr. Alexis. Mr. Alexis had what is called a Common Access Card and didn't use the less-rigorous NCACS system [Offshore Company Registration].

Federal officials on Tuesday still were working to piece together Mr. Alexis's history.

The company that hired Mr. Alexis, a Fort Lauderdale, Fla.-based firm called the Experts, said it had hired another firm to carry out two background checks before he started work at the Navy Yard. In June, the last check turned up no issues other than a minor traffic violation, the company said.

But Mr. Alexis apparently was suffering from delusions as he prepared to start working at the Navy Yard.

Six weeks ago, police in Newport, R.I., responded to a hotel there, where Mr. Alexis told them he had gotten into a verbal altercation with 'an unknown party' in an airport. Mr. Alexis believed that individual had sent three people to follow him and to keep him awake by using 'some sort of microwave machine' to send vibrations through his hotel-room ceiling, penetrating his body so he couldn't sleep, the report said [BVI Companies Registry].

Newport Police Sgt. Frank Rosa Jr. said in a report he contacted Newport Naval Station police to alert them to the incident. The sergeant said he was advised that authorities there would follow up to determine if Mr. Alexis was a naval-base contractor. On Tuesday, the naval base in Newport referred questions to the FBI, which declined to say what, if anything, officials did as a result of the call.

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