President Barack Obama has vowed to see that justice will be done in last week's Fort Hood massacre that left 13 dead and 42 wounded and the alleged shooter will pay for his crime.

"It may be hard to comprehend the twisted logic that led to this tragedy," Obama told an estimated 15,000 people at a memorial service at the Texas post. "But this much we do know: no faith justifies these murderous and craven acts; no just and loving God looks upon them with favour."