Updated 9:25 p.m. EST
A soldier opened fire at a U.S. Army base in Fort Hood, Texas on Thursday, unleashing a stream of handgun fire that left 12 people dead and 31 wounded.
The commander of the Fort Hood Army post, Lt. Gen. Bob Cone, had said in a 5 p.m. news conference that the shooter had been killed, but said at 9:15 p.m. that he had only been wounded and was hospitalized in stable condition.
A Fort Hood press release said that "more than one shooter fired shots into the Soldiers Readiness Processing Center and Howze Theater on Fort Hood" and Cone initially said that eyewitnesses described multiple shooters. But at the 9:15 p.m. news conference he said he believed that there was only one shooter.
Two soldiers were detained and later released. CNN is reporting that a third suspect is still being held.
The shooting began around 1:30 p.m., Cone said. He said all the casualties took place at the base's Soldier Readiness Center, where soldiers who are about to be deployed or who are returning undergo medical screening.
The hospitalized shooting suspect has been identified as Army Maj. Nidal Malik Hasan.
The CBS News investigative unit reports that Hasan, 39, is a licensed psychiatrist who has lived in Bethesda and Silver Spring, Md. and Roanoke, Va. A spokesman for Texas Sen. Kay Bailey Hutchison says Hasan was upset about an upcoming deployment to Iraq.
The victims of the shooting have not been identified but Cone said that the slain included U.S. soldiers and a civilian police officer working at the base. He said that no children had been killed to his knowledge.
"It's a terrible tragedy. It's stunning," Cone said. "Soldiers and family members and many of the great civilians that work here are absolutely devastated."
The wounded are dispersed among several central Texas hospitals and the extent of their injuries varied, Cone said.
President Barack Obama, speaking before a scheduled event in Washington, said, "It's difficult enough when we lose these brave Americans in battles overseas. It is horrifying that they should come under fire at an Army base on American soil."
"I would ask all Americans to keep the men and women of Fort Hood in your thoughts and prayers. We will make sure that we get answers to every single question about this horrible incident," Obama said.
The Army post was locked down until at around 8:30 p.m. Thursday, but Cone said that all of the deaths and injuries had taken place during the initial 1:30 p.m. incident and that authorities were simply exercising and abundance of caution in lifting the lockdown.
FBI agents have responded to the base and the military is asking the FBI's help in investigating the backgrounds of the two soldiers held as suspects in the shooting, Cone said.
A law enforcement official says investigators are trying to determine if Hasan was his birth name, or if he changed his name and converted to Islam at some point in his life.
Military officials say Hasan worked at the Walter Reed Army Medical Center in Washington, DC for six years before being transferred to the Texas base in July. The officials had access to Hasan's military record. They said he received a poor performance evaluation while at Walter Reed.
He is a graduate of Virginia Tech University, where he was a member of the ROTC and earned a bachelor's degree in biochemistry in 1997. He received his medical degree from the military's Uniformed Services University of the Health Sciences in Bethesda, Md., in 2001. At Walter Reed, he did his internship, residency and a fellowship.
During the attack, terrified military families turned to Twitter to find out what was happening, reports CBS News correspondent David Martin.
One tweet read, "locked in my post housing, scared, don't know where the shooters are." Another: "haven't heard warning sirens in about 15 minutes, just waiting to get to my husband. And a third, capturing the shock brought on by today's attack: "i thought i was living in one of the safest places ever."
There has been some confusion about the exact location of the shooting. Cone said that the shooting took place in the Readiness Center but that the two surviving suspects were taken into custody in an "adjacent facility."
A graduation ceremony for soldiers who finished college courses while deployed was going on in an auditorium at the Readiness Center at the time of the shooting, said Lampam said.
Greg Schanepp, U.S. Rep. John Carter's regional director in Texas, was representing Carter at the graduation, a Carter spokesman said.
Schanepp was at the ceremony when a soldier who had been shot in the back came running toward him and alerted him of the shooting, Stone said. The soldier told Schanepp not to go in the direction of the shooter, he said. Stone said he believes Schanepp was in the theater.
"I was here for a 2 p.m. graduation," Schannep, a retired Army colonel, opened fire at a counseling center on a U.S. military base in Iraq, killing five fellow soldiers. showed that that shooter had shown signs of mental problems for months.
And Fort Hood itself has a history of violent incidents, reports CBS News chief investigative correspondent Armen Keteyian.
Around the country, some bases stepped up security precautions, but no others were locked down.
A woman interviewed by CBS affiliate KWTX in Waco shortly after the shooting said that news of the shooting had reached her husband and other U.S. soldiers stationed in Iraq.
At 2 p.m. emergency responders en route to Fort Hood were told that their planned staging area, Soldier's Dome, was not safe to use, the Temple Daily Telegram reports.
Banks said it is too soon to tell whether there is any link to battle stress or repeated deployments. The Army is suffering a record high suicide rate and other signs of stress from fighting two wars.
In a statement, Hutchison said, "I am shocked and saddened by today's outburst of violence at Fort Hood that has cost seven of our brave service members their lives and has gravely injured others. My heart goes out to their loved ones. Our dedicated military personnel have sacrificed so much in service to our country, and it sickens me that the men and women of Fort Hood have been subjected to this senseless, random violence."
Covering 339 square miles, Fort Hood is the largest active duty armored post in the United States. Home to about 52,000 troops as of earlier this year, the sprawling base is located halfway between Austin and Waco.
The base is home to nine schools - seven elementary schools and two middle schools - and all were on lockdown, said Killeen school spokesman Todd Martin.
Texas Department of Public Safety spokeswoman Tela Mange said Texas Rangers and state troopers were en route to Fort Hood to help seal the perimeter of the 108,000 acre base.
Fort Hood officially opened on Sept. 18, 1942, and was named in honor of Gen. John Bell Hood. It has been continuously used for armored training and is charged with maintaining readiness for combat missions.
More coverage of the tragedy at Fort Hood:
Ft. Hood Suspect ID'd as Army Psychiatrist
Soldier Opens Fire at Ft. Hood; 12 Dead
What was Shooter's Motive?
U.S. Army Base Violence Has Bloody History
Obama: Fort Hood Shooting "Horrific"
Fast Facts: Fort Hood
Tragedy at Fort Hood