He yelled “Allahu Akbar!” (God is great) then opened fire on the unarmed military personal waiting to be processed for deployment. He was taken down by a female civilian police office. He’s alive but paralyzed. From USA Today:

An Army psychiatrist who had counseled troops and was upset about being deployed to Iraq shouted “Allahu Akbar!” and opened fire on an unarmed crowd at Fort Hood Army base Thursday afternoon, killing 13 people and wounding 30, military officials said.
Maj. Nidal Malik Hasan, a U.S. citizen born in Virginia to Jordanian parents, was wounded by a civilian police officer responding to a shooting rampage that is believed to be the worst ever at a U.S. base, said Lt. Gen. Robert Cone, commanding general of Fort Hood.
[...]
U.S. Rep. Michael McCaul, a Republican from Austin, was briefed by military officials and said Hasan had taken some unusual classes for someone studying about mental health.
“He took a lot of extra classes in weapons training, which seems a little odd for a psychiatrist,” McCaul said.

USA Today also notes that Hasan may have posted about suicide bombings on the Internet:

Hasan had come to the attention of federal law enforcement officials at least six months ago because of Internet postings that discussed suicide bombings and other threats, according to a federal law enforcement official who spoke to The Associated Press on condition of anonymity because he was not authorized to discuss the case.
The official said investigators were trying to confirm that Hasan was the author of the postings, one of which was a blog that equates suicide bombers with a soldier throwing himself on a grenade to save the lives of his comrades. One of the officials said federal search warrants were being drawn up to authorize seizure of Hasan’s computer.

If he was under suspension, why was he being deployed?
You can also read more about the shootings and response here.
BTW, I agree with this guy that Obama didn’t handle the tragedy in a very presidential way. He knew the nation was awaiting his words and he gives a shout out? Pretty weird.

But instead of a somber chief executive offering reassuring words and expressions of sympathy and compassion, viewers saw a wildly disconnected and inappropriately light president making introductory remarks. At the event, a Tribal Nations Conference hosted by the Department of Interior’s Bureau of Indian affairs, the president thanked various staffers and offered a “shout-out” to “Dr. Joe Medicine Crow — that Congressional Medal of Honor winner.” Three minutes in, the president spoke about the shooting, in measured and appropriate terms. Who is advising him?

Update: Here’s more on the police officer, Sgt. Kimberly Munley, who stopped the Ft. Hood gunman.