Some time after he was discharged from the army, his life turned sour, he got angry and delusional and he snapped. Wielding his gun in a suburban neighbourhood, he killed again and again: women, children, complete strangers, with military precision but without an evident motive...

This describes what police say about John Allen Muhammad, who was arrested yesterday in the Washington sniper-killing rampage. But it is also a precise description of Howard Unruh, a 28-year-old Second World War veteran who shot 13 of his New Jersey neighbours one day in 1949. His military firearms training made his "walk of death" the first modern serial-killer case.

In the 53 years that separate Mr. Unruh from Mr. Muhammad, hundreds of Americans have lost their minds and used guns to cause multiple deaths. One thing unites almost all of them: military training.

Mr. Muhammad served in the Persian Gulf war, as did Timothy McVeigh, the Oklahoma City bomber. Both received weapons training and basic military training designed to psychologically condition soldiers to kill. They are far from alone.

Serial killer Jeffrey Dahmer received military training in Texas and Alabama. David Berkowitz, the "Son of Sam" killer, was an army veteran. Charles Whitman, who killed 16 people and injured 31 in a 1961 sniper-shooting rampage from the top of a tower in Austin, Tex., had just been discharged from the U.S. Marines. Arthur Shawcross, who killed 12 people, was a Vietnam veteran.

In an overwhelming number of cases, serial killers and other mass murderers learned to kill in the military. Experts disagree whether this means that the army turns ordinary people into unfeeling killers, or the military simply attracts a large number of psychologically fragile people who are prone to become murderers.

David Grossman, a former U.S. military psychologist who helped develop programs to train new recruits to become more effective killers, said that the key to military training lies in breaking down the natural human aversion to killing in a process he calls "disengagement." Once this aversion has been removed, it never comes back, and can make it easier for former soldiers to become murderers.

"The ability to watch a human being's head explode and to do it again and again -- that takes a kind of desensitization to human suffering that has to be learned," Mr. Grossman said yesterday.

In earlier wars, many soldiers were psychologically unable to shoot anyone. In order to increase the "trigger-pull ratio," the United States changed the basic training offered to all recruits and draftees so they would be aggressively desensitized to killing.

Some observers believe this may be why mass murders have become far more common in the past 50 years.

In the 1970s, some observers believed that the humiliation and social opprobrium caused by the Vietnam War, led many former soldiers to become mentally unstable, and potentially to become killers.

The Persian Gulf war of 1991 showed that this might not have been the case. Their war was popular domestically, and Persian Gulf veterans were welcomed as heroes upon return.

Yet their conflict has produced more than its share of killers. Mr. McVeigh was a decorated tank commander. His case is strikingly similar to that of Mr. Muhammad.