U.S. President Barack Obama speaks at a memorial ceremony held to honour 13 people killed at a Texas army base. (Kevin Lamarque/Reuters)U.S. President Barack Obama says Americans must never forget the 13 men and women who were killed during a shooting rampage on a U.S. army base at Fort Hood, Texas.

"Neither this country – nor the values that we were founded upon – could exist without men and women like these 13 Americans," Obama said at the memorial service Tuesday afternoon at the army base.

"No faith justifies these murderous and craven acts; no just and loving God looks upon them with favour," he said. "And for what he has done, we know that the killer will be met with justice — in this world and the next."

The president and first lady Michelle Obama began the afternoon by meeting privately with family members of the 12 soldiers and one civilian killed.

The Obamas also met with those wounded in the attack and released from hospital, along with their families, before the president delivered a speech at the outdoor service.

Fort Hood base commander Lt.-Gen. Robert Cone commended the fallen soldiers for their bravery, and thanked the first responders helping the wounded.

"Just by soldiers remembering their training, lives were saved," Cone said. "Some soldiers — many wounded — performed first aid on their battle buddies, before worrying about receiving treatment for themselves."

Charges expected in military court

The lawyer for Maj. Nidal Malik Hasan, the man accused of perpetrating the Nov. 5 mass shooting, said the attention given to the incident will make it difficult for his client to get a fair trial in a military court.

Retired colonel John P. Galligan met with Hasan at the Brooke Army Medical Center in San Antonio on Monday and said he assured Hasan that all of his rights as a defendant in the military justice system will be respected.

Hasan, a psychiatrist at the base, is the only suspect in the shooting rampage, in which an individual opened fire in a room crowded with hundreds of soldiers, killing 13 and injuring 29.

Galligan said Tuesday he thought it would be difficult for Hasan to get a fair trial at Fort Hood, "given the national media attention that has been focused" on the case.

Hasan was shot four times — including at least once in the torso — by two civilian officers and remains in hospital.

Soldiers comfort each other before the start of the memorial service at Fort Hood, Texas. (Donna McWilliam/Associated Press)

Galligan, who was hired by Hasan's family, and Maj. Christopher E. Martin, Fort Hood's senior defence attorney, both met with Hasan after he refused to be questioned by investigators and requested a lawyer on Monday.

Hasan has yet to be formally charged, but officials said he would be tried in a military court, not a civilian one.

The most serious charge in military court is premeditated murder, which carries the death penalty. The U.S. military justice system has not executed anyone since 1961, though five servicemen are currently on death row.

Connection to radical imam

Investigators are reviewing communications between Hasan and a radical imam overseas.

U.S. officials said Monday that Hasan communicated 10 to 20 times with Anwar al-Awlaki, a former imam at the Falls Church, Va., mosque where Hasan and his family occasionally worshipped.

Maj. Malik Nidal Hasan, the U.S. army psychiatrist suspected of perpetrating the shootings, is recovering from gunshot wounds in hospital. (Associated Press)

Al-Awlaki, who was released from a Yemeni jail last year, runs a website that denounces U.S. policies and praised Hasan's alleged actions as heroic.

The FBI has monitored communications with al-Awlaki because of his alleged links to three of the Sept. 11 hijackers, who worshipped at two of his mosques.

The FBI monitored emails exchanged between Hasan and al-Awlaki, but investigators said they had no evidence Hasan had help or outside orders in the shootings.

A law enforcement official told The Associated Press the communications primarily consisted of Hasan posing questions to the imam as a spiritual leader and adviser; the imam responded to at least some of the queries.

Investigators said the content of Hasan's emails was "consistent with the subject matter of his research" as someone who worked with post-traumatic stress disorder cases as a result of combat in Iraq and Afghanistan.

Meanwhile, The Washington Post reported Tuesday that Hasan warned his medical colleagues a year and a half ago that to "decrease adverse events" the U.S. military should allow Muslim soldiers to be released as conscientious objectors instead of fighting in wars against other Muslims.

Hasan made the recommendation in a presentation to senior army doctors at Walter Reed Medical Center, where he spent six years as an intern, resident and fellow before being transferred to Fort Hood.

"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," Hasan said in the presentation, a copy of which was obtained by the Post.

With files from The Associated Press