Major Nidal Malik Hasan, a devout American-born Muslim, was on life support after being shot several times by guards at the Fort Hood base.
Armed with two non-military issue pistols, including a semi-automatic, Hasan, a military psychiatrist, entered a section used to prepare soldiers for deployment overseas and started firing.
Within minutes, 13 soldiers at the base were dead or dying in the worst mass shooting yet at a military base in the US.
At least 30 others - including two civilians - were injured in an attack described by President Barack Obama as an "horrific outburst of violence".
Officials were trying to establish the motive behind the attack.
Retired Colonel Terry Lee, who worked with Hasan, claimed the major was anti-war and argued with comrades who supported operations in Iraq and Afghanistan.
Hasan, a 39-year-old from Virginia, was also due to be sent to Afghanistan, something family members said would have been his "worst nightmare".
Defence sources said at least six months ago Hasan came to the attention of law enforcement officials because of internet postings about suicide bombings and other threats.
They included posts which compared suicide bombers to soldiers who throw themselves on a grenade to save the lives of their comrades.
Investigators were not certain whether Hasan had written the posts and a formal investigation had not been opened before the shooting, said the sources.
Hasan was shot four times but survived and was being treated in hospital under armed guard.
Army spokesman Lieutenant General Bob Cone said he was on a ventilator and unconscious, but added: "I would say his death is not imminent."
His cousin, Nader Hasan, described him as "a good American".
He said the family was "shocked", adding: "We just found out on the news that he was being deployed, he never even told us.
"We've known for the last four or five years that was probably his worst nightmare. He deals with stories, he would tell us how he would hear things, horrific things."
Mr Hasan added that his cousin had also complained of being harassed by others in the military.
Hasan transferred to Fort Hood in July from Walter Reed Medical Centre, where he received a poor performance evaluation, according to an official.
Faizul Khan, a former imam at a mosque Hasan attended in Maryland, said Hasan was a lifelong Muslim and attended prayers regularly, often in his army uniform.
Mr Obama met aides in the White House's Situation Room after being informed of the incident.
The president said his thoughts and prayers were with the wounded and the families of the fallen.
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."
The shootings began yesterday at around 1.30pm local time (7.30pm GMT) at Fort Hood's readiness centre.
The Fort Hood compound - the US's largest military base - was declared safe only five hours later.
Soldiers rushed to treat their injured colleagues by ripping their uniforms into makeshift bandages.
Video footage showed police patrolling the area with handguns and rifles, ducking behind buildings for cover.
Sirens could be heard while a woman's voice on a public address system urged people to take cover.
Soldiers at Fort Hood do not carry weapons unless on training exercises.
Hasan, whose family said he was born in suburban Washington, is single with no children.
In 1997 he graduated with a degree in biochemistry from Virginia Tech, scene of a mass shooting in 2007.
He was a member of the Reserve Officer Training Corps at college.
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