WASHINGTON — Reviews triggered by the Washington Navy Yard killings last year have concluded that 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.

An independent study and an internal review ordered after the September 2013 massacre and released Tuesday said the Pentagon must move beyond an outmoded approach to security that focuses on building better walls and defending the perimeter. More attention must be paid, they concluded, to defending against threats from inside the workforce.

At the same time, however, a Navy investigation also determined that the shooting might have been prevented if the company that employed former Navy reservist Aaron Alexis had told the Navy about problems it was having with him in the months before he gunned down 12 civilian workers in a Navy Yard building.

“The most effective methods to prevent targeted violence in the workplace must be employed long before someone enters an installation with a weapon,” the internal review said.

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, but then restored it two days later and never told the Navy about it.

The broader department reviews reached similar conclusions and recommendations, including that the department cut the number of workers who hold security clearances, conduct better and routinely updated background checks, and set up a system to evaluate and handle employees who may pose a threat.

While the reviews were ordered as a result of the Navy Yard shootings, they reflect similar worries brought out by the massive intelligence leaks by former National Security Agency contract systems analyst Edward Snowden.

“Threats to our personnel and classified information increasingly lie within our installations, and come from DoD employees and contractors who are trusted insiders,” said the independent review. “The Department of Defense needs to strengthen security from within, and reframe its policies and programs to counter insider threats.”

Marcel Lettre, the principal defense undersecretary for intelligence, said the department is looking at phasing in a system that would do continuous evaluations of employees who hold security clearances.

And Defense Secretary Chuck Hagel said the Pentagon will consider cutting the number of workers with active security clearances — currently about 2.5 million — by 10 percent.