Thirteen people are dead and 30 injured after a US army major opened fire on fellow soldiers at Fort Hood military base in Texas.
The attacker has been identified as 39-year-old military psychiatrist Major Nidal Malik Hussein.
He 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.
He was shot four times by a policewoman, and is now in a stable condition in hospital. Earlier reports indicated that he had been killed.
Base Commander Lieutenant General Bob Cone said that one of the dead was a policeman and the others were soldiers.
Speaking at a press conference in Washington, President Barack Obama condemned the attack. "It is difficult enough when we lose these brave men and women abroad, but it is horrifying that they should come under fire at an army base on US soil," he said.
Offering condolences to the family, he said: "We will make sure that we get answers to every single question about this horrible incident."
It is not clear what motivated Virginia-born Major Hasan, although there have been other violent incidents across the US involving troops suffering from stress on return from war zones.
Major Hasan was reportedly due to be sent on a mission to Iraq or Afghanistan, a deployment which his cousin said he had been resisting.
Nader Hasan told Fox news: "He hired a military attorney to try to have the issue resolved, pay back the government, to get out of the military. He was at the end of trying everything."
He added that Major Hasan had undergone racial harassment about his Middle Eastern ethnicity and Muslim background.
The Pentagon warned against speculation as to motive. Pentagon press secretary Geoff Morrell said: "I don't know how anybody can speculate about motives at this time given how few facts we have."
Before working at Fort Hood, Major Hasan served as a psychiatrist at the Walter Reed Army Medical Center in Washington, treating wounded troops from combat zones.