Since the wars in Iraq and Afghanistan began earlier this decade, soldier-on-soldier violence has been relatively rare. Before Thursday's shooting at Fort Hood, Texas, fewer than a dozen soldiers are believed to have fallen victim to such attacks since 9/11.

But prominent incidents in the past six years have illustrated that motivations for such violence can fall into several categories. One includes a soldier whose mental health was called into question after repeated deployments to war zones, and another a convert to Islam whose motives for attacking fellow soldiers were political.

U.S. military officials said Thursday that they are still investigating the motives of Maj. Nidal Malik Hasan, the Army officer identified by authorities as the man who killed 12 in a shooting spree at Fort Hood. One official said he had expressed concern about a coming deployment to the Middle East.

Just six months ago, a 44-year-old Army sergeant, a veteran of tours in Serbia, Bosnia and Iraq, was charged with the murder of five fellow soldiers at a military counseling center in Baghdad, where he had been sent after others in his unit had expressed concerns about his mental health.

According to military officials familiar with the case, the soldier, Sgt. John M. Russell, had surrendered his weapon to military authorities after members of his unit warned superiors that Sgt. Russell was behaving dangerously. Despite being disarmed, officials said Sgt. Russell managed to grab the gun of another soldier who was escorting him out of the building.