FORT HOOD, Texas (CNN) -- Army investigators on Sunday asked troops and civilians for help in its probe of a deadly mass shooting at Fort Hood last week, saying some who fled the gunfire might have evidence.

"The Fort Hood office of the U.S. Army Criminal Investigation Command is seeking any military or civilian personnel who may have left the scene ... with gunshot damage such as damaged privately owned vehicles, personnel clothing, etc.," investigators said in a written statement. "CID is also seeking any military or civilian personnel who may have inadvertently left the scene of this incident with material that could be used as firearms residue related evidence such as shell casings inside the boot, etc."

The statement said such objects would help Army investigators and the FBI "in their bullet trajectory analysis of the scene, to insure the comprehensiveness of the ongoing investigation."

Thirteen people -- a dozen soldiers and a civilian -- died Thursday in the shooting at the Fort Hood Army post. Some 42 people were wounded, according to the post's public information office. It was unclear how many of those suffered gunshot wounds.

As of Saturday night, 17 people remained hospitalized plus the suspect, Maj. Nidal Malik Hassan, spokesman Col. John Rossi told reporters. He did not say where all those hospitalized were being treated.

Rossi said Hasan, a 39-year-old licensed Army psychiatrist who worked at a hospital on the post, is no longer on a ventilator, but is still in intensive care at Brooke Army Medical Center.

W. Roy Smythe, chief of surgery at Scott & White Memorial Hospital, said progress had been made in treating the victims at that facility, and some were "out of the woods."

Army Criminal Investigation Command spokesman Chris Grey said that all evidence indicates that the suspect acted alone and that there was no evidence of "friendly fire."

He said authorities have not identified a motive in the attack at Fort Hood's military processing center, where soldiers report before they head to war.

Hasan's brother, Eyad Hasan, released a statement saying the family was in a "state of shock and disbelief over this dreadful news."

He wrote, "I've known my brother Nidal to be a peaceful, loving and compassionate person who has shown great interest in the medical field and in helping others. He has never committed an act of violence and was always known to be a good, law-abiding citizen."

Eyad Hasan said the family has faith in the legal system.

An earlier statement from another family member said Hasan, a U.S.-born citizen of Palestinian descent, had been telling his family that he wanted to get out of the military but had been unsuccessful in doing so.

Rossi said Saturday that Hasan had a late November deployment date to Afghanistan. It was to have been his first deployment.

The shooting, on the nation's largest military base, sparked outrage. In his Saturday radio address, President Barack Obama said it was "an act of violence that would have been heartbreaking had it occurred any place in America."

But, he said, "it's all the more heartbreaking and all the more despicable because of the place where it occurred and the patriots who were its victims."

The president had cautioned earlier against "jumping to conclusions" about what had triggered the shooting.

Obama and first lady Michelle Obama will attend a Tuesday memorial service, the White House said. The president has ordered flags flying over the White House and other federal buildings to be lowered to half-staff until Veterans Day, which is on Wednesday.

Texas Gov. Rick Perry, speaking Saturday outside the Scott & White hospital, lauded the hospital's quality and professionalism and praised the patriotism of the soldiers.

"What I heard time after time, in those hospital rooms: that it's their honor to be able to serve our country," he said.

Perry added that it was humbling to watch "a young man or woman whose life has been irreparably harmed in a violent act" have their concern and interest lie in "continuing to be able to serve this country."

The governor praised the first responders and mentioned Fort Hood Police Sgt. Kimberly Munley, the civilian officer who confronted and disabled Hasan in a shootout.

Perry spoke with Munley by phone Friday, and called her a "true professional" and a "selfless public servant."

She's "a person who understands the gravity of what occurred, but also a classic public servant who is not interested in anything but getting on with her life and hopefully never having an event like this ever occur again," he said.

Munley has drawn praise from the military and from citizens across the nation for her quick and bold actions.

Rossi read a statement from Munley and her family on Saturday, saying they are "deeply touched," by the nation's thoughts and concerns.

"Kimberly underwent her second surgery this morning and is currently in good condition," the statement said, according to Rossi.

Obama and Perry both said the tragedy brought out the best in people, citing the efforts of soldiers and civilians to aid others.

"Even as we saw the worst of human nature on full display," the president said, "we also saw the best of America."

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