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.
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.
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.