US President Barack Obama said no faith could justify the Fort Hood rampage, in an address Tuesday memorializing victims of the accused gunman, Major Nidal Hasan, a devout Muslim.

"It may be hard to comprehend the twisted logic that led to this tragedy," Obama said in remarks to be delivered at a memorial service after last week's tragedy which were released by the White House.

"But this much we do know -- no faith justifies these murderous and craven acts; no just and loving God looks upon them with favor.

"And for what he has done, we know that the killer will be met with justice -- in this world, and the next."

This video is from CNN's Newsroom, broadcast Nov. 10, 2009.

Questions mounted Tuesday over the accused Fort Hood gunman's links to militant Islam as President Barack Obama headed for a memorial service and to console bereaved relatives at the base.

Intrigue over troubled military psychiatrist Major Nidal Hasan deepened after the FBI revealed he had contacts with a firebrand Islamic cleric in Yemen and it emerged he had voiced doubts over the role of US Muslim soldiers.

Obama, shouldering the US president's role as leader of national mourning and commander-in-chief, left the White House early Tuesday with his wife Michelle. Both looked somber and were dressed in black as they boarded his Marine One helicopter.

The president was set to address the memorial service and meet privately with grieving relatives of 12 service personnel and one civilian killed in the bloody shooting spree on the base last week which also left 42 wounded.

"The president will use the opportunity to honor 13 men and women, to talk a little bit about each of them and to discuss their contributions they made," his spokesman Robert Gibbs said aboard Air Force One.

Obama's trip comes just two weeks after he consoled grief-stricken relatives as fallen US soldiers were flown home from Afghanistan. Vice President Joe Biden was performing a similar role at a service in Washington state Tuesday.

New details began to emerge Tuesday on Hasan's past.

The FBI said Hasan, a devout Muslim, came to its attention in 2008 after he communicated with the target of an FBI-led counter-terror investigation.

The bureau said investigators assessed the contacts were "consistent with research being conducted by Major Hasan in his position as a psychiatrist at the Walter Reed Medical Center.

"The JTTF (joint terrorism task force) concluded that Major Hasan was not involved in terrorist activities or terrorist planning," the FBI said.

The FBI added "the investigation to date indicates that the alleged gunman acted alone and was not part of a broader terrorist plot."

The Washington Post reported investigators were examining possible links between the army psychiatrist and Anwar al-Aulaqi, who is now in Yemen but was a spiritual leader of the Dar al-Hijrah mosque in Falls Church, Virginia.

Hasan had attended the mosque in 2001.

The imam was said to have met Al-Qaeda associates, including two September 11 hijackers, and is now believed to have become a supporter of the terror network, the paper said, citing a senior US official.

The Post also said Hasan had shocked fellow army medics more than a year ago by saying that Muslim soldiers should be allowed release as conscientious objectors rather than being ordered to go to war against fellow Muslims.

"It's getting harder and harder for Muslims in the service to morally justify being in a military that seems constantly engaged against fellow Muslims," he was quoted as saying in a presentation.

After regaining consciousness, Hasan has been able to talk for the first time since he allegedly opened fire on fellow soldiers until he was shot by a female police officer.

The shootings have set off nationwide soul-searching and worries about the motives of the Fort Hood gunman. Related article: More Fort Hoods waiting to happen

In a television interview on the eve of his visit to the base, Obama said the question being asked was: "'Is this an individual who's acting in this way or is it some larger set of actors?'"

Gibbs said Obama had asked US government agencies to investigate why and how the incident happened and to ensure "that it won't happen again."

FBI Director Robert Mueller, after meeting with Obama, ordered a full review of the incident with the aim of determining whether "with the benefit of hindsight, any policies or practices should change based on what we learn," the FBI said.

The bloody spree left army officials still scrambling to understand how one of their own could turn on his fellow soldiers.

Officers must now keep an eye out for similar signs of disquiet "across our entire formation, not just in the medical community, but look hard to our right and left," said Lieutenant General Robert Cone, the Fort Hood commander.