U.S. President Barack Obama participated in a memorial service at the Fort Hood Army base, Texas, Tuesday to honor 13 people killed by a U.S. Army psychiatrist.

Obama honored the men and women killed in Thursday's shooting rampage for their service, and said that the fact the they were killed on American soil makes "the tragedy even more painful."

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

"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," the president said.

The president recalled memories of those killed. "Neither this country - nor the values that we were founded upon - could exist without men and women like these thirteen Americans. And that is why we must pay tribute to their stories," he said.

The men and women killed at Fort Hood embody responsibility "in an age of selfishness" and "in an era of division, they call upon us to come together. In a time of cynicism, they remind us of who we are as Americans," the president said.

Obama also vowed that justice will be done. "We are a nation of laws whose commitment to justice is so enduring that we would treat a gunman and give him due process, just as surely as we will see that he pays for his crimes."

Tuesday's memorial ceremony includes prayers, speeches, a sermon and a "roll call" tribute to the 13 dead.

U.S. Defense Secretary Robert Gates, Army Chief of Staff George Casey, Texas governor Rick Perry and more than a dozen members of Congress were among the roughly 15,000 estimated to have attended the service before the III Corps building at Fort Hood.

In front of the podium, the boots, helmets and rifles of the slain soldiers sat behind each of their service photographs.

Some 545 soldiers from Fort Hood have been killed in the wars in Iraq and Afghanistan, said Robert Cone, the post's commander. "But never did we expect to pay such a high price at home."

"No words can fully address your grief and sadness," Cone told the families. "But it is important to know you are not alone."

Obama delayed his departure for Asia by a day to make the ceremony service.

The president and his wife, Michelle, arrived in Killeen before noon Central time. Before attending the ceremony, the Obamas visited privately with the families of those killed.

The shooting, the worst in history at a U.S. military base, took place 1:30 p.m. CST (1930 GMT) Thursday at a family readiness area at Fort Hood, the country's largest military installation. Thirteen people were killed and another 29 were wounded.

The suspect in the shooting, Nidal Malik Hasan, a Army mental health doctor, is going to be deployed overseas, very likely in Afghanistan and he "was disturbed" about his overseas deployment, military sources said.