KILLEEN, Texas, (Reuters) – Investigators searched  for the motive yesterday 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.

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

“We don’t know all the answers yet and I would caution  against jumping to conclusions until we have all the facts,”  Obama said.

Hasan’s cousin, Nader Hasan, said in interviews that he had  agitated not to be sent overseas. “We’ve known over the last  five years that was probably his worst nightmare,” he said.

Nader Hasan also said his cousin had complained, as a  Muslim, of harassment by fellow soldiers.