Secretary of Defense Chuck Hagel discussed what measures the Department of Defense will take in light of the attack on Washington's Navy Yard, which led to the deaths of 13 people. (The Associated Press)

The Defense Department is likely to reduce the number of employees who hold security clearances by at least 10 percent and has vowed to overhaul the way it screens personnel, officials said Tuesday as they released the results of several inquiries into the Sept. 16 mass shooting at the Navy Yard.

The reviews offered a damning assessment of the department’s ability to monitor the trustworthiness and reliability of a workforce that grew exponentially in the aftermath of the Sept. 11, 2001, attacks. They also made clear that the Pentagon has issued security clearances to many employees and contractors who are not required to access classified information in the course of their jobs.

Investigators found that Navy personnel and supervisors who later employed gunman Aaron Alexis as a defense contractor “missed opportunities for intervention” that could have barred the former sailor from retaining a secret security clearance and unfettered access to military installations.

“The reviews identified troubling gaps in the Department of Defense’s ability to detect, prevent and respond to instances where someone working with us decides to inflict harm on this institution and its people,” Defense Secretary Chuck Hagel said Tuesday morning.

Despite the numerous lapses outlined in the two internal assessments and an independent review, the Pentagon said it has not disciplined or fired anyone as a result of the case, the second deadliest mass shooting on a U.S. military installation.

Read eyewitness accounts from the Navy Yard shooting.

“We hold Aaron Alexis accountable,” said Cmdr. Amy Derrick-Frost, a Pentagon spokeswoman. The gunman was fatally shot by police, ending the rampage.

The reports offered no new substantive clues about the motives of Alexis, who was reported to have exhibited paranoid and delusional behavior in the weeks before he walked into a Navy Yard building and gunned down 12 Navy civilian personnel and contractors.

The independent panel commissioned by the Pentagon concluded that the department’s procedures to protect sensitive information and installations are outdated and must better take into account security threats posed by insiders.

“Threats increasingly are from within,” Paul N. Stockton, one of the experts commissioned by the Pentagon to conduct an independent review, said in an interview. “There needs to be a systemic solution.”

The panel also urged the department to take a hard look at the number of employees who actually need to access classified information for their jobs, noting that since the Sept. 11 attacks, the number of clearances approved annually has tripled.

The shooting has reinvigorated efforts to develop a system in which individuals with security clearances would be screened continuously based on up-to-date information such as police records and arrest reports. Currently, the department relies mainly on reviews that are conducted every five or 10 years.

A pilot program that recently screened 3,370 soldiers, civilians and contractors against government databases and other ­sources suggests that the type of enhanced screening system the Pentagon hopes to standardize could yield troubling information about a high percentage of the workforce, according to the report.

Investigators found red flags on about 731 individuals in that sample, including 99 with “serious derogatory information” that included financial issues, domestic abuse, drug use and prostitution. The review led the Pentagon to yank the clearances of 55 employees and the access to military facilities of the remaining 44.

In the case of the Navy Yard shooting, the first missed opportunity came when the Office of Personnel Management, which conducts the bulk of security clearance reviews for federal workers, screened Alexis in 2007. The agency’s report included “multiple discrepancies that went undetected and unchallenged,” the Pentagon’s main review found. Investigators did not learn details of instances in which Alexis was arrested for violent episodes involving firearms. He also failed to report the extent of his debt and an overseas trip.

Those oversights appear to be fairly routine. The main Pentagon review notes that 31 percent of background investigations of prospective or current defense employees conducted by the Office of Personnel Management are assessed as “incomplete or inadequate.” In many instances — including Alexis’s — security clearances are nonetheless granted.

Three years before he left the Navy in 2011, Alexis’s supervisor recommended that the sailor not be retained, classifying him as a service member with “significant problems.” Despite that finding and a subsequent arrest, Alexis was given an honorable discharge and Navy officials did not follow procedures to document a pattern of “chronic” troubling behavior.

That enabled Alexis to keep his secret clearance, which later helped him land a job as a computer technician with the Experts Inc. None of the derogatory information about his time in the Navy was relayed to the firm, the Pentagon’s review found. But the reviews faulted the Experts for failing to seek assistance from mental health professionals or guidance from the Pentagon when colleagues became alarmed by Alexis’s delusions and paranoia.

The firm did not respond to a phone call and e-mail requesting comment.

Stan Soloway, the president and chief executive of the Professional Services Council, a trade organization that represents government contractors, said that security clearance reform has been needed for years and that continuous monitoring of cleared workers “is absolutely critical.”

“It’s the right thing to do,” he said. “But the implementation is going to be a challenge.”

The culture of the investigations would need to be radically reformed, he said, with an increased reliance on technology.

“We have a process now that is so heavily dependent and focused on interviews and shoe leather,” he said.

Evan Lesser, the managing director at ­, said it could take years to fully implement meaningful ­changes.

“We have 5 million cleared professionals,” he said. “They’re working all over the globe. Continuous monitoring sounds like a great idea, but from an implementation standpoint it’s going to be costly and difficult.”

Some relatives of Navy Yard victims were disappointed by the Pentagon’s reports. Sidney Matthew, an attorney for the family of victim Mary Frances DeLorenzo Knight, called them a “whitewash” and questioned why no one has been held accountable.

“This was negligence by people,” he said. “There should be heads rolling.”

Matt Zapotosky and Marc Fisher contributed to this report.