Fort Hood, Texas (CNN) -- The sound of taps echoed across the Texas plains Tuesday after President Obama pledged that the work of those killed in last week's Fort Hood massacre will go on despite their "incomprehensible" slayings.

Speaking to an estimated 15,000 people at a memorial service at the post, Obama vowed that justice will be done in the attack that left 13 dead and 42 wounded.

Though he told the families that "no words can fill the void that has been left," he added, "your loved ones endure through the life of our nation."

"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.

After his remarks, Obama and first lady Michelle Obama laid a presidential coin before each of the 13 battlefield crosses -- the helmet, boots and rifle representing each of those killed -- before family members and comrades filed past.

Fort Hood Army Post has seen 545 soldiers killed in the wars in Iraq and Afghanistan, said Lt. Gen. Robert Cone, the post's commander, "but never did we expect to pay such a high price at home."

Gen. George Casey, the Army's chief of staff, added, "Grieve with us. Don't grieve for us."



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"Those who have fallen did so in the service of their country," he said. "They freely answered the call to serve, and they gave their lives for something that they loved and believed in."

Obama called the wartime killings of American troops on their home soil "incomprehensible." But he said the values the dead volunteered to defend will live on and will be extended even to the man accused of carrying to the slayings.

The suspected gunman in the attack is a 39-year-old Army psychiatrist, Maj. Nidal Malik Hasan, who remained in intensive care at an Army hospital in San Antonio, Texas.

Hasan, an American-born Muslim of Palestinian descent, was scheduled to deploy to Afghanistan but had told his family that he wanted to get out of the military.

"No faith justifies these murderous and craven acts," Obama said at the memorial service. But he said soldiers who responded to the attack "remind us of who we are as Americans."

"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," he said.

No charges have been filed, and authorities have not identified a motive in Thursday's attack. But in a statement issued Monday night, the FBI said its investigation "indicates that the alleged gunman acted alone and was not part of a broader terrorist plot."

Thursday's victims included 12 soldiers and a retired soldier working as a civilian physician's assistant.

Shortly before the ceremony and 1,200 miles away, the remains of one of the soldiers was carried off a chartered jet in Milwaukee, Wisconsin.

An honor guard met the casket of Sgt. Amy Krueger on the apron at General Mitchell International Airport.

Krueger, 29, was a high school athlete who joined the military after the September 11 attacks on New York and Washington. She was assigned to a medical unit that was doing checkups on soldiers bound for Afghanistan and Iraq when the shooting erupted.

Defense Secretary Robert Gates, Texas Gov. Rick Perry and more than a dozen members of Congress were among who attended the service on the warm Texas afternoon.