The massacre at Fort Hood that left 13 dead and 38 injured was a "kick in the gut," Army Chief of Staff Gen. George W. Casey Jr. said Friday, as military officials tried to piece together what led an Army psychiatrist to open fire on his fellow soldiers.

The Army wouldn't discuss a motive, but initial reports suggested that the suspect, 39-year-old Nidal Malik Hasan, was dreading his imminent deployment to Afghanistan and had been critical of the wars there and in Iraq.

Thursday's mass shootings are believed to be the worst ever on a military base in the United States. Hasan was among those wounded and remained hospitalized in a coma Friday.

"This is a time for the Army family to stand together," said Secretary of the Army John McHugh, calling it an "extraordinarily tough day" and acknowledging that there will be "challenging times ahead."

Authorities were investigating what pushed a man who helped troubled soldiers cope with the aftermath of war to turn on his fellow soldiers.

"I'll tell you candidly, this was a kick in the gut," Casey told reporters at an afternoon briefing held jointly with McHugh.

SLIDESHOW: Deadly Fort Hood Rampage

At a news conference late Friday, Army Col. John Rossi, deputy commander at Fort Hood , said 23 people remained hospitalized, about half still in intensive care. He praised the soldiers' quick actions during and after the shooting barrage, which he said saved lives.

Rossi said that the assailant fired more than 100 rounds and that his weapons were not military arms, but "privately owned weapons ... purchased locally." Law enforcement sources in Washington , speaking on condition of anonymity, said records indicate Hasan in recent months bought the FN 5.7 pistol at a store called "Guns Galore" in Killeen , Texas .

Several hundred people gathered at a base stadium where the Army's chief chaplain offered prayers for families and victims of the shooting rampage.

Chaplain Douglas Carver told those at the Friday night vigil — many dressed in fatigues and black berets — to "remember to keep breathing ... keep going."

God Bless America and Amazing Grace were sung as husbands wrapped their arms around their wives, babies cried and old men in wheelchairs bowed their heads.

It was the first community gathering since the killings.

A moment of silence was held Friday afternoon at Fort Hood and other U.S. military bases as a show of respect for the victims.

"There are family, friends and an entire nation grieving right now for the valued men and women who came under attack yesterday in one of the worst mass shootings ever to take place on an American military base," President Obama said in brief remarks at the White House.

About 20 different units were affected by the shooting, according to Casey.

"I've asked our leaders around the country to look at their installations, to look at what's going on and to adjust their force protection measures to deal with it," Casey said. "I do worry slightly about a backlash" against Muslim soldiers in the Army, he said.

McHugh singled out the 36th Engineer Brigade as especially hard hit, with four killed and 11 wounded.

McHugh said officials have not determined whether a case would be pursued in federal or military court and that no charges have been filed yet. Officials are not ruling out the possibility that some of the casualties may have been victims of "friendly fire," shot by responding military officials.

Suspect's Computer, Trash Can Seized

Earlier Friday, FBI agents seized Hasan's computer and took away a trash bin as his family denounced the crime he is accused of committing as "despicable and deplorable."

It was not immediately known if FBI agents found anything suspicious on Hasan's computer files that might explain the rampage.

A military official said investigators also are sifting through materials Hasan carried with him during the massacre and other evidence left in his vehicle, which was found parked at the base.

Survivors of the rampage said the suspect shouted "Allahu Akbar!" — "God is great!" in Arabic — before opening fire, according to Fort Hood base commander Lt. Gen. Robert Cone. Cone said officials had not yet confirmed that Hasan made the comment.

"That was reported by a few of the witnesses that were at the scene," Cone's deputy, Rossi, told Fox News.

Authorities searched his apartment early Friday for clues but haven't yet been able to talk to Hasan, who survived the shootings but is in a coma in a Texas hospital, according to a federal law enforcement official.

He was due to be sent any day to the war zone in Afghanistan to counsel soldiers there suffering from combat stress, Army officials said.

Army spokeswoman Col. Cathy Abbott said Hasan, an active-duty military psychiatrist, was to deploy with an Army Reserve unit that provides what the military calls "behavioral health" counseling.

It wasn't immediately clear whether Hasan sought the assignment or was being sent against his wishes.

"He was mortified by the idea of having to deploy,” Hasan's cousin Nader Hasan told The New York Times. “He had people telling him on a daily basis the horrors they saw over there.”

Hero Police Officer Stopped Spree

A wounded female civilian police officer who shot Hasan four times, identified as Kimberly Munley, was credited with stopping the spree. Munley fired on the suspect just three minutes after the gunfire erupted.

The gunman then spun around and charged at Munley with a gun in each hand, said her boss, Chuck Medley, director of emergency services at Fort Hood. He said Munley shot Hasan in the upper torso, allowing officers to take him into custody. Munley, who was shot in the thighs and wrist, was recovering Friday at a hospital.

"It was an amazing and an aggressive performance by this police officer," Cone said.

President Obama said authorities were working to figure out how the tragedy could have happened — but warned against rushing to judgment.

"We don't know all the answers yet and I would caution against jumping to conclusions until we have all the facts," Obama said.

In honor of those mourning the victims, Obama said he ordered all flags at the White House and other federal buildings to be flown at half staff until Veteran's Day, Nov. 11.

The wounded were dispersed among hospitals in central Texas. W. Roy Smythe, chairman of surgery at Scott & White Memorial Hospital, said some of the wounded have "extremely serious injuries" and several patients were still at "significant risk" of losing their lives.

The dead included a pregnant woman who recently returned from a tour of duty in Iraq, a man who quit a furniture company job to join the military about a year ago and a woman who had vowed to take on Usama bin Laden after the Sept. 11 terrorist attacks.

Kim Fuller, a spokeswoman for the Hasan family, issued a statement Friday saying "the actions of their cousin are despicable and deplorable."

Relatives in Northern Virginia are trying to offer insight to investigators, she said, adding that Hasan's suspected crime doesn't reflect how he was raised in the United States.

"We are trying to make sense of all this," the suspect's cousin told Fox News. "He wasn't even someone who enjoyed going to the firing range."

Suspect Had 'Difficulties' at Work

Hasan, who was born in Northern Virginia, pursued a career in psychiatry at Walter Reed, working as an intern, a resident and a fellow in disaster and preventive psychiatry.

But his record there wasn't sterling. He received a poor performance evaluation, according to an official who spoke on condition of anonymity because he was not authorized to discuss the case publicly. And while he was an intern, Hasan had some "difficulties" that required counseling and extra supervision, said Dr. Thomas Grieger, who was the training director at the time.

One of Hasan's bosses praised his work ethic and said he provided excellent care for his patients.

"Up to this point I would consider him an asset," said Col. Kimberly Kesling, deputy commander of clinical services at Darnall Army Medical Center.

In an interview with The Washington Post, Hasan's aunt, Noel Hasan of Falls Church, Va., said he had been harassed about being a Muslim in the years after the Sept. 11, 2001, terror attacks, and he wanted out of the Army.

"Some people can take it and some people cannot," she said. "He had listened to all of that and he wanted out of the military."

At least six months ago, Hasan came to the attention of law enforcement officials because of Internet postings about suicide bombings and other threats, including posts that equated suicide bombers to soldiers who throw themselves on a grenade to save the lives of their comrades.

Investigators had not determined for certain whether Hasan was the author of the posting, and a formal investigation had not been opened before the shooting, said law enforcement officials who spoke on condition of anonymity because they are not authorized to discuss the case.

U.S. Muslims reacted with both anger and fear of backlash after revelations that Hasan is a practicing Muslim. The nation's major Muslim organizations and several mosques condemned the attacks as contrary to Islam.

"The community is in a state of agony," said Muqtedar Khan, director of the Islamic studies program at the University of Delaware and a well-known progressive Muslim intellectual.

Some U.S. mosques stepped up security on Friday, the main prayer day for Muslims.

Hasan was working with soldiers at Darnall Army Medical Center on Fort Hood after being transferred in July from Walter Reed Army Medical Center, where he had worked for six years before recently receiving a poor review.

Hasan was not known to be a threat or risk, Fort Hood officials said.

"We had no problems with job performance while he was working at Darnall," said Col. Steve Braverman, Fort Hood's hospital commander. When pressed, Braverman added, "Not here. ... I'm not aware of any problems here."

Gunman 'Calm' During Shootings

Cone said witnesses spoke of the methodical way Hasan conducted the rampage, what he called a "very calm and measured approach" in carrying out the mass shooting. About 300 soldiers were lined up to get shots and eye-testing at a Soldier Readiness Center when gunfire erupted about 1:30 p.m. Thursday.

"It seems like a very high number of people for a single shooter," Cone told Fox News Friday. "This is a very small area. ...In talking to the soldiers last night who were present, the shooter could move in very close distances and fire at very close range and hit a number of people."

All but two of the injured — 28 total — were still hospitalized, including Hasan, and all were in stable condition, Braverman told reporters.

When the gunfire subsided, soldiers described a scene that looked like a war zone: too many wounded to count, shells and blood on the floor, and comrades ripping off their clothes to make tourniquets to keep the injured alive. One woman, suffering from a wound to the hip, carried another victim to get help.

"You had people without tops on. You had people ripping their pant legs off," said Sgt. Andrew Hagerman, a military police officer from Lewisville, Texas, who arrived at the scene minutes after the shooting stopped.

A neighbor at the apartment building near Fort Hood where Hasan lived said they had recently discussed his impending deployment to Afghanistan.

"He seemed OK with it," said Edgar Booker, a 58-year-old retired soldier who now works in a cafeteria on the post. "I asked him how he felt about going over there, with their religion and everything, and he said, 'It's going to be interesting.'"

Neighbors described a man who appeared to be getting his affairs in order just hours before the shooting. Before Thursday's shooting, Hasan gave away all of his furniture along with copies of the Koran to friends who lived nearby, one neighbor said.

Jacqueline Harris, 44, said Hasan called her boyfriend, Willie Bell at 5 a.m. Thursday.

"He just wanted to thank Willie for being a good friend and thank him for being there for him," Harris said. "That was it. We thought it was just a nice message to leave."

In the early chaos after the shootings, authorities believed they had killed the gunman, only to discover later that he had survived. A police source told FoxNews.com that Hasan's vital signs failed while he was being transported by ambulance to the hospital after the rampage but he was revived.

Retired Army Col. Terry Lee, who said he worked with Hasan, told Fox News that Hasan had hoped Obama would pull troops out of Afghanistan and Iraq. Hasan got into frequent arguments with others in the military who supported the wars, Lee said, and had tried hard to prevent his pending deployment.

FoxNews.com's Jana Winter, Catherine Donaldson-Evans, Michelle Maskaly and The Associated Press contributed to this report.