A police officer runs near the scene of the shooting rampage at the Washington Navy Yard on Monday, September 16. Authorities said at least 12 people -- and the suspect -- were killed in the shooting. 
A U.S. Park Police officer stands guard near the scene of the shooting. 
Military security observes the scene from a nearby rooftop. 
The White House flies the U.S. flag at half-staff following the shooting. 
A police boat patrols the waters at the Washington Navy Yard. 
Late Monday afternoon the FBI identified Aaron Alexis, a 34-year-old military contractor from Texas, as the dead suspect involved in the shooting. 
Workers who had been sheltered during a lockdown exit the Navy Yard. 
People step off buses at Parking Lot B of Nationals Park, which was set up as a gathering point for family members of Navy Yard employees in the wake of the shooting. 
A man and woman embrace at the gathering point inside Nationals Park. 
A man walks with a woman after they were reunited at Nationals Park. 
A woman reunites with her child inside Nationals Park. 
Washington Mayor Vincent Gray and Washington Police Chief Cathy Lanier speak to the media near the Washington Navy Yard. 
Police officers walk on a rooftop at the Washington Navy Yard. 
A member of the military stands guard at the scene of the shooting. 
A woman watches from an office building as police respond to the shooting. 
Police officers leave the scene after responding to the shooting. 
A police officer and police canine inspect vehicles in a parking lot outside the Washington Navy Yard. 
A Homeland Security officer tells a woman where to park and wait for her husband, who works in the Washington Navy Yard. 
A police helicopter hovers above snipers on the roof of a building at the Washington Navy Yard. 
Police stand guard on M Street in Washington. 
Police boats patrol the Anacostia River off the Washington Navy Yard complex. 
A U.S. Capitol Police officer keeps watch on the East Plaza of the Capitol in Washington. 
Troops stand guard in front of a parking garage near the Washington Navy Yard. 
Police vehicles move toward the Navy Yard. 
Armed agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives work on the 11th Street Bridge adjacent to the Washington Navy Yard. 
Emergency vehicles fill the streets around the Washington Navy Yard. 
A helicopter lifts a person off a roof at the Navy Yard. 
A police officer carries an automatic rifle at the Washington Navy Yard. 
Police officers adjust their gear. 
Police block off a road leading to the Washington Navy Yard. 
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- Peter Bergen: The Washington Navy Yard shootings not an isolated case
- There has been a string of cases of veterans and service people attacking military targets
- Bergen says it's good that Defense Department is reviewing flaws in security
- Bergen: One issue that slipped through the cracks was status of contractors
Editor's note: Peter Bergen is CNN's national security analyst, a director at the New America Foundation and the author of "Manhunt: The Ten-Year Search for bin Laden -- From 9/11 to Abbottabad."
(CNN) -- Aaron Alexis, the troubled civilian contractor and Navy veteran who killed a dozen fellow workers this week at the Washington Navy Yard, is far from the only veteran or active-duty serviceman to plan or carry out mayhem directed at U.S. military targets during the past decade.
It's a deadly combination: men who have military backgrounds -- together with personal grievances, political agendas or mental problems -- and who also have easy access to weapons and are trained to use them.
As a result of Alexis' deadly rampage, U.S. Defense Secretary Chuck Hagel on Wednesday announced a review of access procedures at all U.S. military facilities and also the procedures for granting security clearances to Department of Defense employees, including civilian contractors such as Alexis.
Until Alexis' attack Monday, Maj. Nidal Hasan, a Palestinian-American, was the most well-known case illustrating the "insider threat" at U.S. military facilities.
On November 5, 2009, Hasan, an Army psychiatrist, opened fire on soldiers at Fort Hood in Killeen, Texas, killing 13 and wounding many others.
Hasan had been communicating via e-mail with the militant Yemeni-American cleric Anwar al-Awlaki about the permissibility of killing fellow soldiers. He had specifically cited the case of Sgt. Hasan Akbar, who while he was deployed to Kuwait in 2003, threw grenades at fellow soldiers, killing two officers and wounding more than a dozen others.
Akbar had complained to his father about religious and racial harassment before the attack and had been diagnosed with some kind of mental illness when he was a teenager. Akbar was convicted and sentenced to death. The sentence is under appeal.
More than a year after Hasan's massacre of his fellow soldiers, Army Pvt. Naser Jason Abdo also plotted to kill servicemen at Fort Hood. Abdo had applied for a conscientious objector status discharge because of his Muslim faith and cited Hasan's 2009 attack as an influence on his own murderous plot.
Hagel: We'll fix security gaps 
How can lawmakers ensure security? Abdo's plot was foiled in July 2011 after an employee at a local gun store tipped off authorities that Abdo was behaving suspiciously.
After the Fort Hood attack and a number of reviews examining the lessons learned from it, then-Defense Secretary Robert Gates issued a directive in 2010 ordering the implementation of 47 recommendations to improve "force protection" at military bases in the United States.
These recommendations included giving military personnel better guidance on the behavioral indicators of potentially violent servicemen, establishing a consolidated criminal investigation and law enforcement database for the Department of Defense, and re-evaluating background check policies.
All of these policies might have flagged Alexis as someone to watch given his long history of run-ins with law enforcement and obvious mental health issues.
However, the recommendations by Gates coming out of the Fort Hood shooting specifically did not address policies governing contractors such as Alexis.
This was a missed opportunity because civilian contractors now number around 900,000 at the Defense Department. That's almost double the number of active-duty personnel in the entire Army.
In addition to Monday's attack at the Navy yard, 2013 has seen other deadly incidents involving insiders at U.S. military facilities.
In March, Eusebio Lopez, a Marine sergeant, shot and killed two other Marines at the Marine base in Quantico, Virginia, and then committed suicide. The shootings may have been the result of a romantic dispute, according to a senior Pentagon official who spoke on background to The Washington Post.
In July, Tech. Sgt. Matthew Hullman, who worked at the Grand Forks Air Force Base in North Dakota, confronted security personnel while holding a handgun and was killed. The incident is still under investigation.
There may not be any one answer to the problem of preventing "insider" attacks targeting the military. But there's hope that the review of base access procedures and security clearances just now ordered by Hagel will reduce the number of such violent incidents at U.S. military facilities in the future.
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