NEW YORK —
And yet, twice in less than four years, a person with permission to be there passed through the layers of protection at a US base and opened fire, destroying the sense of security at the installations that embody the most powerful military in the world.
‘‘It is earth-shattering. When military bases are no longer safe, where is safe?” said Colonel Kathy Platoni, a reservist who keeps a gun under her desk after witnessing the shooting at Fort Hood in Texas in 2009, when Major Nidal Hasan, an Army psychiatrist, killed 13 people.
In the wake of this week’s deadly rampage at the Washington Navy Yard, Defense Secretary Chuck Hagel ordered the Pentagon to review security at all US defense installations worldwide and examine the granting of security clearances that allow access to them.
‘‘We will find those gaps and we will fix those gaps,’’ Hagel vowed on Wednesday.
After Fort Hood, the military tightened security at bases nationwide. Those measures included issuing security personnel long-barreled weapons, adding an insider-attack scenario to their training, and strengthening ties to local law enforcement, said Peter Daly, a vice admiral who retired from the Navy in 2011. The military also joined an FBI intelligence-sharing program aimed at identifying terror threats.
Then, on Monday, Aaron Alexis, a 34-year-old former Navy reservist who held a security clearance as an information technology employee at a defense company, used a valid pass to get into the Washington Navy Yard and killed 12 people before dying in a gun battle with police.
Despite the apparent concerns over his mental health and past run-ins with the law, Alexis maintained his security clearance as he arrived in Washington in late August for a position as an information technology employee at a defense-related computer company.
He used a valid badge to gain access to the sprawling Navy Yard and Building 197, bringing with him a shotgun bearing the cryptic messages of ‘‘better off this way’’ and ‘‘my ELF weapon,’’ according to a law enforcement document reviewed by The Associated Press. The meaning of those words wasn’t immediately clear.
Authorities say Alexis had with him during the massacre a handgun he picked up from an officer inside the building and a legally obtained Remington 870 Express shotgun — a firearm that would not be covered under a proposed weapons ban supported by the White House.
The attack has raised questions about the adequacy of the background checks done on government contractors who hold security clearances.
Hagel acknowledged ‘‘a lot of red flags’’ may have been missed in the background of the gunman, who had a history of violent behavior and was said to be hearing voices recently.
Many of the security improvements adopted after 9/11 and Fort Hood were created largely with terrorism in mind, not unstable individuals with no apparent political agenda. Those threats can be more difficult to detect.
Daly, who directs the US Naval Institute in Annapolis, Md., said the military needs to review its procedures for vetting people for access to installations.
‘‘Once you’re inside that hardened line of defense, that is the most difficult scenario,’’ he said. ‘‘We need to look at how these clearances are granted to contractors and subcontractors and to make sure once someone is granted clearance, that we come back and check again.’’
Some of the shock and sudden sense of vulnerability caused by Fort Hood and the Navy Yard attack may have stemmed from the mistaken belief that military personnel are armed when they are on domestic installations.
Most personnel are, in fact, barred from carrying weapons onto a base, and Hasan and Alexis probably knew it. Another little-known fact is that many searches are random. Not all vehicles or packages are checked.
In Southern California, Marine Captain Aaron Meyer said it would be impossible to eliminate all threats without making military operations too costly or inefficient. In 2010, he said, his parents were let in through the main gate at Camp Pendleton after guards checked only his father’s driver’s license, even though his mother was a passenger in the car.
John Barney, owner of Tri-Star Commercial, an Austin, Texas, security company that has put cameras and card access systems in several military installations, said that after Fort Hood, the Pentagon mostly responded by increasing the armed police presence, but added few electronic measures.
But he admitted electronic security is not necessarily enough.
Just recently, he said, he was in a military warehouse area near San Antonio and entered without showing any identification or encountering military police. In one building, he came across an open metal cage stacked with M-16 rifles that anyone could have walked off with, he added.