WASHINGTON — 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 say the shooting by a Navy contractor could have been prevented if the company that employed Aaron Alexis told the Navy about problems it was having with him in the months before he gunned down 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 for homeland defense and one of the authors of the independent review. “That approach is outmoded, it’s broken, and the department needs to replace it.”

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. The Associated Press reported those findings late last year.

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