The shootings at the Army's Fort Hood in Texas Thursday were an "isolated incident," according to military officials.

But there have been other attacks by military personnel in recent years – some in the United States, some in the war zone in Iraq – and they happened over a period of sustained combat and multiple deployments when increasing numbers of soldiers are experiencing Post-Traumatic Stress Disorder (PTSD) and committing suicide.

Officials are just beginning to learn the relevant details about Maj. Nidal Malik Hasan, who killed 11 people and wounded 31 before being killed himself. He was a medical doctor and a licensed psychiatrist. The shootings took place at Ft. Hood's "Soldier Readiness Center," where troops are helped to prepare for deployment. Early media reports suggest that Hasan had been heard complaining about orders to deploy and that he had been heard complaining about that.

"This is an isolated and tragic case, and we're obviously in the process of obtaining more information as the events unfold," said Defense Department spokesman Lt. Col. Eric Butterbaugh.

Attacks on fellow soldiers – particularly officers – was dubbed "fragging" in Vietnam.

Between 1969 and 1971, the Army reported 600 fragging incidents that killed 82 Americans and injured 651, according to the Associated Press. In 1971 alone, there were 1.8 fraggings for every 1,000 American soldiers serving in Vietnam, not including gun and knife assaults.

Such incidents have dropped dramatically. But in recent years there have been several incidents in the United States and Iraq. As tallied by NBC News and the Associated Press, they include:

• May 11, 2009: Five soldiers shot dead at Camp Liberty in Baghdad by Sgt. John Russell.

• Sept. 8, 2008: Spc. Jody Michael Wirawan shoots himself to death after killing 1st Lt. Robert Bartlett Fletcher at Fort Hood.

• Feb. 25, 2008: Dustin Thorson, an Air Force technical sergeant, fatally shoots his son and daughter at home on Tinker Air Base in Oklahoma in domestic dispute with ex-wife. He had been diagnosed with post-traumatic stress disorder after returning from Iraq.

• June 7 2005: Two National Guard officers are killed by a grenade at headquarters in Tikrit. Staff Sgt. Alberto Martinez is later acquitted of murder in a court-martial.

• March 23, 2003: Army Sgt. Hasan Akbar tosses grenades into three tents and then fires a rifle at Camp Pennsylvania in Kuwait, killing one and injuring 14. Akbar later receives death sentence.

• Oct. 27, 1995: Sgt. William Kreutzer goes on shooting spree at Fort Bragg, killing one and injuring 18 in a sniper attack during formation. He is serving a life sentence after a death sentence was overturned.

Although the military services have done much in recent years to address the effects of a lengthy war on service personnel and their families, multiple deployments and the stress of combat have taken their toll.

Researchers report that the rate of PTSD and other mental difficulties tied to war may be as high as 35 percent.

The Army reported in January that the rate of suicide among soldiers had increased in 2008 for the fourth year in a row.

"At least 128 soldiers killed themselves last year, and the Army suicide rate surpassed that for civilians for the first time since the Vietnam War, according to Army statistics," The New York Times reported.

See also:

Soldier rampage hints at stress of repeated deployments

'Surge home' overwhelms Veterans Affairs clinics

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