US President Barack Obama led his nation in mourning Saturday as shocked Americans struggled to understand why a Muslim army doctor killed 13 in a massacre at a US military base.

Alleged shooter Major Nidal Malik Hasan, 39, a psychiatrist and specialist in combat stress who had been about to deploy to Afghanistan against his wishes, also wounded 30 people in Thursday's rampage.

Speculation swirled at Fort Hood, Texas as to whether the alleged shooter had snapped under the pressure of his job counseling thousands of war-weary troops, or was motivated by deeper convictions.

In his weekly radio address Saturday, Obama sought to reassure US soldiers.

"Thursday's shooting was one of the most devastating ever committed on an American military base," he said. "And yet, even as we saw the worst of human nature on full display, we also saw the best of America."

The president noted that soldiers and civilians rushed to help, tearing off bullet-riddled clothes to treat the injured and using blouses as tourniquets.

Obama ordered flags to fly at half-staff at the White House and federal buildings, as troops here and around the world held a minute's silence to mourn the dead.

Obama would also attend a memorial service due to be held in the coming days, the White House said.

The bodies of those killed were taken to the same mortuary at Dover Air Base in Delaware that handles fallen soldiers from the wars in Afghanistan and Iraq, the Pentagon said Friday.

Hasan was moved meanwhile from a civilian to a military hospital, in part for security reasons, Fort Hood deputy commander Colonel John Rossi told reporters.

HASAN WAS YELLING ALLAHU AKBAR AND EVENTUALLY STOPPED BY HEROINE OFFICER

Army chief of staff General George Casey said the attack was "a kick in the gut, not only for the Fort Hood community but for the entire army."

Hasan was shot and seriously wounded by a female civilian police officer who was being hailed as a heroine for ending his deadly rampage.

Thursday's shooting was one of the most devastating ever committed on an American military base and yet, even as we saw the worst of human nature on full display, we also saw the best of America.

Barrack Obama

Witnesses reportedly heard Hasan shout "Allahu Akbar!" (God is greatest) as he opened fire in a troop processing center with a semiautomatic weapon and a handgun.

Rossi said investigators believe Hasan fired more than 100 rounds during the incident.

Although "Allahu Akbar" is a Muslim prayer, it has come to be associated with Islamic militants as they carry out attacks or suicide bombings.

A surveillance video aired by CNN showed the major buying breakfast wearing traditional Muslim garb at a base store just hours before the shooting.

The New York Times reported that on Wednesday and Thursday, Hasan seemed in a hurry to give his belongings, including a copy of the Koran, to a neighbor.

"I'm not going to need them," he told the neighbor, Patricia Villa, according to The Times.

MAJOR NIDAL MALIK HASA ALREADY KNOWN TO GOVERNMENT INVESTIGATORS?

The newspaper also reported that an early search of Hasan's computer did not indicate any direct exchanges with known terrorists.

But earlier this year, the Times report said, the Federal Bureau of Investigation became aware of Internet postings by a man calling himself Nidal Hasan that favorably discussed suicide bombings.

However, the investigators were not able to determine yet whether the writer and Major Hasan were the same person, the paper noted.

The bloodshed dealt a new blow to US forces already under severe strain from repeated combat tours and plagued by a rise in suicides and depression.

Fort Hood, by area the world's largest US military base, has borne the brunt of the wars in Iraq and Afghanistan. Troops based here have suffered the highest number of casualties and have undertaken multiple tours of duty.

The shooting also raised delicate questions about Muslim soldiers serving in the US Army, as some Muslim groups feared a huge backlash.

Casey, the army chief of staff, said after a visit to the base that he, too, feared that possibility.

Hasan was born in the United States to Palestinian parents who had moved from a small town near Jerusalem.

His cousin Nader Hasan, writing on behalf of the family as Hasan's parents are dead, said they were stunned by Thursday's events and stressed they all considered themselves Americans.

"Our family loves America. We are proud of our country, and saddened by today's tragedy," Nader Hasan said in the message posted on The Washington Post website.

Nidal Hasan's aunt, Noel Hasan, told the daily her nephew had been subjected to harassment about his faith since the September 11, 2001 attacks and had repeatedly sought to be discharged.