The Navy Yard Shooting Could Have Been Prevented, Review Finds

But the review includes one major, disconcerting caveat: "Even if those vulnerabilities had not been present, neither the personnel security process nor the physical security capability is equipped or designed to prevent the kind of violence exhibited by Aaron Alexis."

Following his service in the Navy, Alexis was treated on several occasions for psychological issues. He also had a couple of run-ins with the law for illegally discharging his weapon. While the Pentagon concedes that, individually, these events were not enough to predict this outcome, together they are damning.

These instances, the review says, should have been detected by the background investigation before he received his security clearance. His employer at the time of the shooting, a defense contracting company called the Experts, had no insight into these behaviors, nor did the company report his psychological issues to the Defense Department, the report continues.

There were also gaps in security at the Washington Navy Yard, the report shows, as random inspections for vehicles and bags did not meet Defense Department standards.

"There is no way to know, however, whether more frequent inspections might have given law-enforcement personnel the opportunity to discover the weapon Alexis carried onto the installation and neutralize or minimize the immediate threat," the report says.

Plans to cut security presence at military bases, however, "are likely to leave the department vulnerable to threat."

In light of recent fatal shootings at U.S. military bases, including when Maj. Nidal Hasan killed 13 people in 2009, the Pentagon has looked into ways of preventing this kind of violence long before the shooting begins. The review recommends more evaluations of cleared Defense Department personnel using "automated records checks and reports of behavior of concern and recommend action as appropriate," while also increasing a security presence at military buildings.

A separate independent review also recommends new security installations at military buildings, while also cutting the number of Defense Department employees and contractors with security clearance by 10 percent. The review further notes that there should be "more and better" data for clearance checks, and increased mental health care awareness.

Defense Secretary Chuck Hagel said Tuesday the department would close the "troubling gaps" with the security of contractors, military, and security personnel.