"We come together filled with sorrow for the 13 Americans that we have lost, with gratitude for the lives that they led, and with a determination to honour them through the work we carry on," Obama yesterday said addressing a crowd of 15,000 soldiers and their families at the Fort Hood army base.

Speaking out victims' names softly, one by one, Obama with a heavy heart, narrated their stories, and denounced the "twisted logic" that led to their deaths.

"It may be hard to comprehend the twisted logic that led to this tragedy. But this much we do know. No faith justifies these murderous and craven acts; no just and loving God looks upon them with favour.

"And for what he has done, we know that the killer will be met with justice, in this world, and the next," said Obama, dressed in black and accompanied by his wife Michelle.

Meanwhile, reports here claimed that 39-year-old army psychiatrist Nidal Malik Hasan, who slipped into coma after being shot by a woman police officer following his shooting spree at the Fort Hood military base on Thursday last, has contacts with an al-Qaeda-linked radical cleric.

"This is a time of war. Yet these Americans did not die on a foreign field of battle. They were killed on American soil, in the heart of this great state and the heart of this great American community. This is the fact that makes the tragedy even more painful, more incomprehensible," Obama said.

"For those families who have lost a loved one, no words can fill the void that's been left. We knew these men and women as soldiers and caregivers. You knew them as mothers and fathers, sons and daughters, sisters and brothers," he said.

Obama said their memory will be honoured in the places they lived and by the people they touched, and promised a full investigation of the shootings.

"Their life's work is our security, and the freedom that we too often take for granted. Every evening that the sun sets on a tranquil town; every dawn that a flag is unfurled; every moment that an American enjoys life, liberty and the pursuit of happiness -- that is their legacy," he said.

The president and first lady met privately with family members of those killed and with those wounded in the attack and their families. The shooting killed 12 soldiers and one civilian and injured 29 others.

Below the stage where Obama spoke was a sombre tribute to the fallen - 13 pairs of combat boots, each with an inverted rifle topped with a helmet. A picture of each person rested below the boots. After the ceremony, Obama walked solemnly along the row of boots, placing a commander-in-chief's coin next to each victim's photo in tribute. Mourners then traced the same path to give a final salute to them.

Even as Obama honoured the dead, there was government finger-pointing over what had been known about Hasan and whether he should have been investigated further.

US officials said a Pentagon worker on a terrorism task force had looked into Hasan's background months ago and had concluded he did not merit further investigation.

Two officials said the group had been notified of links between Hasan and the radical cleric Anwar al-Awlaki and the information had been turned over to a Defence Criminal Investigative Service employee assigned to the task force.

The revelation provided fresh evidence that the military had been keeping an eye on the Army psychiatrist before the shooting rampage.

One soldier, who attended the memorial, said the mood at Fort Hood was turning from sadness to anger as soldiers learned more about Hasan's background.