FORT HOOD, TEXAS - Investigators try to figure out the motive behind the worst-ever massacre on a United States army base. 

Witnesses said that the shooting suspect, Nidal Malik Hasan, 39, walked into a processing center and yelled, "Allahu Akbar!" before opening fire.  That's Arabic phrase meaning, "God is great!"

That happened at 1:30 p.m. CT Thursday.  Officials said there was one shooter.

Army officials said Hasan killed 12 soldiers and one civilian during his shooting spree.  He wounded 30 more people.  Victims said he was methodical. 

One said that after he got shot four times, and when he moved, Hasan shot him again, so he stopped moving. 

Lt. Gen. Robert Cone said at one point Hasan realized that many soldiers escaped.  That's when he walked outside and started shooting.

Hundreds of unarmed soldiers were inside that facility at the time.  They don't carry weapons on base. 

Civilian Police Officer Kimberley Munley was responded within a minute.  She fire at Hasan four times.  Hasan fired back and shot her once. 

Officials said Munley is okay.  She's still in the hospital, but she's talking on the phone with family and friends.  Hasan is in stable condition on a ventilator.