WASHINGTON — 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 investigations into the Sept. 16 mass shooting at the Washington 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 wake 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,’’ Secretary of Defense 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 US military installation.
‘‘We hold Aaron Alexis accountable,’’ said Commander 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 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by the Pentagon concluded that the department’s procedures to protect sensitive information and installations are outdated and must better take into account security threats posed by insiders.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 Stockton, one of the experts commissioned by the Pentagon to conduct the 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 9/11 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 Army soldiers, civilians, and contractors using 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 the 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.