Earlier reports throughout the day said he'd been killed by law enforcement.
The Army just announced that the gunman, Army Major Nidal Malik Hasan, was shot but survived, and is in the hospital in stable condition.
Two other soldiers who were detained have been released, but another person of interest is in custody, said Christopher Haug, chief of public affairs at Fort Hood.
Hasan is a psychiatrist, licensed in Virginia. He attended Virginia Tech University and earned advanced degrees in psychiatry and public health. He previously worked at Walter Reed Army Medical Center, and was more recently practicing at Darnall Army Medical Center at Fort Hood, according to professional records. Tonight the Washington Post is reporting that Hasan is an American citizen and a devout Muslim, born in Virginia to parents who'd immigrated from the Middle East.
Hasan was scheduled to be deployed to Iraq "and appeared to be upset about that," Senator Kay Bailey Hutchison, R-Texas, said.
"I think that there is a lot of investigation going on now into his background and what he was doing that was not known before," Hutchison said.
Ten of the other dead also were soldiers, while one was a civilian police officer who was working as a contractor on the base.
Initial reports said the gunman had two weapons, both handguns. The Army now says the shots were fired by some kind of semi-automatic weapon.
Senator Hutchison said she was told that the soldiers at the readiness facility "were filling out paper processing to go to Iraq or Afthanistan," according to CNN affiliate KXAN in Austin, Texas.
President Obama called the shootings "tragic" and "a horrific outburst of violence." He expressed condolences for the shooting victims.
"These are men and women who have made the selfless and courageous decision to risk, and at times give, their lives to protect the rest of us on a daily basis," Obama 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."
Fort Hood is the Army's largest U.S. post. With about 40,000 troops, it is home to the Army's 1st Cavalry Division and elements of the 4th Infanty Division, as well as the 3rd Armored Cavalry Regiment and the 13th Corps Support Command. Fort Hood is also home to the Warrior Combat Stress Reset Program, which is designed to help soldiers overcome combat stress issues.
The base is next to Killeen, Texas, site of one of the worst shooting rampages of the last century. In 1991, a man named George Hennard drove his pickup truck into the Luby's Cafeteria in Killeen, then systematically gunned down 23 people and wounded another 20. He shot and killed himself as police moved in.