An army psychiatrist mowed down twelve of his own colleagues at Fort Hood military base in Texas. Thirty one others were wounded.
The killer, Major Nidal Malik Hasan, was himself shot by a policewoman, but his condition is said to be stable.
Virginia-born Hasan opened fire with two handguns at the base's soldier readiness centre, where troops were having equipment checks, dental treatment and other last-minute preparations before being flown to Iraq.
The base, the biggest US active facility in the world, covering 339 sq miles and home to 52,000 troops at any one time, was locked down, with all troops told to remain in barracks and civilians told to stay at home.
A civilian who lives close to the soldier readiness centre said: "We were told to close our doors. It has just been crazy. There were lots of firing."
The rampage has alarmed the US establishment, already receiving a lot of flak for failure to respond adequately to the mental health needs of its soldiers.
Though stories of soldiers cracking under pressure are legion, Hasan himself was not a soldier returning from deployment in either Iraq or Afghanistan, suffering from stress or combat fatigue. Hasan, although 39 years old, had never served in a war zone.
Instead, his horror of war came second-hand. He was a psychiatrist who listened to the harrowing stories of his comrades at the Walter Reed Hospital in Washington DC, and latterly at Fort Hood, Texas.
He was brought up at Roanoke, in rural Virginia, and then went to Virginia Tech, where he joined the Officer Training Corps. From there, he went into the military and trained as a psychiatrist.
He was promoted in May and transferred from Walter Reed to Fort Hood. He was said to be single, with no children.
Unusually for a soldier, Hasan appeared to have little taste for violence, at least up until yesterday. His cousin, Nader Hasan, said: "He was someone who did not enjoy going to the firing range." That may have been a consequence of the stories he had heard in the hospital wards from the returning soldiers.
Hasan had suffered harassment from fellow soldiers who questioned his loyalty to the US and commented on his Middle East ethnicity. As a Muslim, he was upset at the killing of Muslims in Iraq and Afghanistan, and he had been resisting deployment in either war zone.
His problem may have been one of alienation, as his family suggested yesterday, a common complaint of recent or second-generation immigrants, as was the case of the Virginia Tech shooter three years ago, whose family was South Korean.
A retired colonel, Terry Lee, who worked beside Hasan in a ward, said he had been unhappy about US foreign policy and had made several comments that the US should not be in Iraq and Afghanistan.
He had been scheduled for deployment to Iraq at the end of the year and had told colleagues repeatedly he did not want to go. He felt trapped, looking at ways to buy his way out, even going to the extent of hiring a lawyer to seek if he could leave military service honourably, Ewen MacAskill wrote for the Guardian.
Back in 2003 too, another Muslim soldier, Sergeant Hasan Akbar, had thrown a grenade into a tent of his comrades in Kuwait. He was sentenced two years later to death and the prosecution claimed he had been motivated by Islamist extremism. Still it did not blow up into any major furore within or outside the army, perhaps because the incident was overshadowed by the Iraq invasion.
It could be different now, though the Pentagon is trying to play it safe, not rushing to attribute the Texas carnage to any religious rage.
The US military is dealing with a rising number of stress-related homicides and suicides among soldiers either serving in Iraq and Afghanistan, or who have returned from duty in those conflicts. Many troops are on their third or fourth combat tour.
Some studies suggest that about 15% of soldiers returning from Iraq suffer from emotional problems.
Last year there were 128 confirmed suicides by serving US army personnel, and 41 by serving marines - the highest number since records began in 1980.
Another 15 army deaths were still under investigation when the figures were released in February.
The confirmed rate of army suicides was 20.2 per 100,000 last year. In 2002, the army suicide rate was just 9.8 per 100,000. The last time it exceeded the civilian rate was at the height of the Vietnam war.
Paul Sullivan, executive director of Veterans for Common Sense, said that he and others had warned that an incident such as the one which unfolded yesterday had been on the cards for a long time. "We warned the military about this. We warned the military about the need to increase the number of mental health care providers...."
Ironical in that Hasan himself was supposed to have been one such mental healthcare provider.