UPDATE -- 2:34 a.m.
Officials said that soldiers and civilians ripped apart their clothes to make bandages for fallen colleagues, many of whom were waiting at the base's Soldier Readiness Center for medical and dental exams before deployment. The attack shocked the country and raised questions about base security.
The suspected gunman, who initially was thought to have died, was wounded and in stable condition under guard at a hospital. Identified as Maj. Nidal Malik Hasan, 39, he worked at the Darnall Army Medical Center, Ft. Hood's hospital. The facility has an extensive program to help soldiers deal with the stress of returning from war.
Base commander Lt. Gen. Robert Cone said at a news conference Thursday evening that Hasan was shot multiple times by a female civilian Army police officer, who was also injured. The suspect reportedly had not spoken with investigators, and Cone would not say anything more about him.
A senior U.S counter-terrorism official said Thursday night that the Army and FBI were looking into whether Hasan, who is Muslim, had previously come to the attention of federal law enforcement officials as the suspected author of inflammatory Internet comments likening suicide bombers to heroic soldiers who give their lives to save others.
The official, who spoke on condition of anonymity because the investigation was ongoing, said that authorities would examine Hasan's actions in the months leading up to the rampage in part to determine whether authorities had missed warning signs. "This is going to be a long and convoluted and messy investigation," the official said.
Although three other soldiers were briefly taken into custody, Cone said he believed that the gunman acted alone.
President Obama lamented the attack as a "horrific outburst of violence" and promised justice. "We are going to stay on this," he said.
"These are men and women who have made the selfless decision" to protect the nation, Obama said of the victims. "It is horrifying that they should come under fire at an Army base on American soil."
Hasan, a Virginia native, worked at Walter Reed Army Medical Center for six years before his transfer to the Texas base in July. Army officials with access to Hasan's records told the Associated Press that he had received a poor performance evaluation at Walter Reed.
In a post on the website scribd.com that appears to be from May, a writer named "NidalHasan" likened a suicide bomber to a soldier who jumps on a grenade to save the lives of his fellow officers in that both were sacrificing their lives "for a more noble cause."
That cause, he wrote, "is to help save Muslims by killing enemy soldiers. If one suicide bomber can kill 100 enemy soldiers because they were caught off guard that would be considered a strategic victory. Their intention is not to die because of some despair. The same can be said for the Kamikazees in Japan."
Sen. Kay Bailey Hutchison (R-Texas) told NBC News that Hasan was about to be deployed to combat for the first time "and was upset about it." Hasan's cousin, Nadar Hasan, a lawyer in northern Virginia, told Fox News that deployment was his cousin's "worst nightmare."
The shooting broke out about 1:30 p.m. Central time at the Soldier Readiness Center. About 50 meters away, Cone said, 138 soldiers were preparing to go through a 2 p.m. college graduation ceremony before 600 guests.
"Thanks to the quick reaction of several soldiers," Cone said, "they were able to able to close off the doors to that auditorium." That action, he said, saved lives.
Cone said he did not believe that the weapons involved in the attack -- one of them a semiautomatic -- were military-issued.
The rules for carrying weapons on an Army post are standard throughout all bases, service officials said. The only personnel allowed to openly display weapons on the base are military police, said Lt. Col. Nathan Banks, an Army spokesman.