US President Barack Obama on Saturday sought to reassure US service men and women in the wake of a deadly shooting rampage in Fort Hood, Texas, saying the tragedy had also displayed "the best of America."

"Thursday's shooting was one of the most devastating ever committed on an American military base," Obama said in his weekly radio address.

"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 alike had rushed to help fallen comrades, tearing off bullet-riddled clothes to treat the injured, using blouses as tourniquets and taking down the shooter even when injured themselves.

Alleged shooter Major Nidal Malik Hasan, 39, a psychiatrist and specialist in combat stress who was about to deployed to Afghanistan against his wishes, went on a shooting rampage at Fort Hood on Thursday, killing 13 people and wounding 30.

Speculation swirled around the base on Friday as to whether Hasan had snapped under the pressure of his job counseling thousands of war-weary troops, or was motivated by deeper convictions.

Obama said he had met Friday with Federal Bureau of Investigation Director Robert Mueller, Defense Secretary Robert Gates and representatives of other federal agencies to discuss their investigation into what had led to this crime.

He promised to stay in close contact with them as new information came in, but stressed that it might not be possible to understand what had motivated such an attack.

"But what we do know is that our thoughts are with every single one of the men and women who were injured at Fort Hood," he said.

"Our thoughts are with all the families who've lost a loved one in this national tragedy.

"And our thoughts are with all the Americans who wear - or who've worn - the proud uniform of the United States of America; our soldiers, sailors, airmen, marines, and coast guardsmen, and the military families who love and support them."