A US major believed to have killed 13 people in a gun attack at a Texas army base was due to be deployed soon to Afghanistan, a military official said.
Maj Nidal Malik Hasan, 39, a US-born Muslim, opened fire on fellow soldiers at Fort Hood on Thursday.
Relatives of the army psychiatrist said he had strongly opposed his deployment and had wanted to leave the army.
US officials said investigations into what prompted the attack had continued through the night.
Early on Friday the commander of the Fort Hood base, Lt Gen Robert Cone, told NBC News that, according to eyewitnesses, the gunman had shouted the Arabic phrase "Allahu Akbar!" [God is great] before opening fire.
Military officials said 12 soldiers and one civilian had been killed.
Of the 28 people who remained in hospital, 14 had required surgery but all were in a stable condition, Col Dr Steven Braverman said.
Deputy base commander Col John Rossi declined to comment on what might have triggered the attack. "We'll let investigators find that out," he said.
Maj Hasan was shot four times during the attack and is currently being treated in hospital under armed guard.
The policewoman who shot him first - named as Kimberly Munley, 34 - was among those wounded.
President Barack Obama described the shooting as "a horrific outburst of violence".
He said: "It is difficult enough when we lose these brave Americans in battles overseas. It is horrifying that they should come under fire at an army base on American soil."
source:BBC