WRAPUP 5-Motive probed for US army shooting rampage

* Suspected gunman was psychiatrist, had treated wounded

* Psychiatrist was shot four times but stable

* Soldiers were shot at pre-deployment center (Adds details from Army officials, color)

By Erwin Seba

KILLEEN, Texas, Nov 6 (Reuters) - Investigators searched for the motive on Friday behind a mass shooting at a sprawling U.S. Army base in Texas, in which an Army psychiatrist trained to treat war wounded is suspected of killing 13 people.

The suspected gunman, Major Nidal Malik Hasan, a Muslim born in the United States of immigrant parents, was shot four times by police, a base spokesman said. He was unconscious but in stable condition.

A woman died overnight from her wounds, raising the toll from Thursday's shootings to 13 dead and 30 wounded, said Colonel John Rossi, a spokesman at Fort Hood, the biggest military facility in the world.

Hasan was on a ventilator in a civilian hospital, Rossi said.

The Army refused to discuss possible motives while the investigation was under way. "We have to understand what caused the suspect to act in the way that he did," Army Secretary John McHugh said after observing a moment of silence at the base.

"This was a kick in the gut," said Army Chief of Staff George Casey.

The gunman, with two guns including a semi-automatic weapon, opened fire apparently without warning at the crowded Soldiers Readiness Processing Center, where troops were getting medical checkups before leaving for foreign deployments.

Hasan, 39, had spent years counseling severely wounded soldiers at the Walter Reed Army Medical Center in Washington, many of whom had lost limbs fighting in Iraq and Afghanistan.

He was transferred to Fort Hood in April and was to have been deployed to Afghanistan, where the U.S. military is engaged in an increasingly bloody war against Taliban and al Qaeda fighters.

The Army Criminal Investigation Command and the FBI are investigating the shootings and no charges have been brought against Hasan, McHugh said.

DEPLOYMENT 'NIGHTMARE'

In Washington, President Barack Obama warned met with with FBI officials, including agency director Robert Mueller, to discuss the incident.