– Threats to Defense Department personnel and facilities increasingly are coming from trusted insiders, and to defeat them the Pentagon must beef up security from within, according to several reviews triggered by last year's Washington Navy Yard killings.

The reviews said the shooting by a Navy contractor could have been prevented if the company that employed Aaron Alexis had told the Navy about problems it was having with him in the months before he shot 12 civilian workers.

An independent study and an internal review ordered after the September 2013 massacre and released Tuesday said the Pentagon must expand its focus beyond defending against external threats. More attention must be paid, they concluded, to defending against threats from inside the workforce.

"For decades, the department has approached security from a perimeter perspective," said Paul Stockton, former Pentagon assistant secretary and one of the authors of the independent review. "That approach is outmoded, it's broken, and the department needs to replace it."

Navy never notified

According to the Navy probe, the Fort Lauderdale, Fla.-based company, The Experts, pulled Alexis' access to classified material because of concerns he was having mental health problems. It then restored his access two days later and never told the Navy about it.

Alexis, a former Navy reservist, was shot to death during the incident.

The department reviews said the Pentagon should cut the number of workers who hold security clearances, conduct better and routinely update background checks, and establish a system to evaluate and handle employees who are potential threats.

The Navy investigation's most damning charges were against Alexis' employers.

The report written by Navy Adm. John Richardson said Alexis' behavior raised concerns among his supervisors and others, and indicated he may harm others. Had such information been reported to the government and acted upon, it said, Alexis' authorization to secure facilities would have been revoked.

Alexis' company temporarily withdrew his access to classified information after a series of bizarre complaints and police incidents last August. Alexis complained that people were following him and using a microwave to "send vibrations through the ceiling" in his hotel room.

The report said The Experts' human resources manager called Alexis' mother, who said her son "has been paranoid and this was not the first episode he had experienced."

Access was restored

The Experts concluded that the information on Alexis was based on rumor and it restored his access. His security clearance from the Navy carried over when he went to work as a contractor.

Defense Secretary Chuck Hagel said Tuesday the department will set up an automated program that will continuously pull information from law enforcement and other databases. It will send out alerts if damaging information about a security-cleared worker is discovered.

Hagel said an inside threat management center will analyze the record checks and "help connect the dots." He will consider cutting the number of workers with clearances — currently about 2.5 million — by at least 10 percent.

The Pentagon may also take over background checks for its workers, which are now done by the federal Office of Personnel Management. Hagel said the department will look at the costs.