The gunman, who served in Iraq for four months in 2011, had been undergoing an assessment before the attack to determine if he had post-traumatic stress disorder, Milley said.
He arrived at Fort Hood in February from another base in Texas. He was taking medication, and there were reports that he had complained after returning from Iraq about suffering a traumatic brain injury, Milley said. The commander did not elaborate.
The gunman was never wounded in action, according to military records, and there was no indication the attack was related to terrorism, Milley said. His weapon had been recently purchased in the local area and was not registered to be on the base, Milley said.
Those injured were taken to the base hospital and other local hospitals. At least three of the nine patients at Scott & White Memorial Hospital in Temple were listed in critical condition.
Wednesday's attack immediately revived memories of the 2009 shooting rampage on Fort Hood, the deadliest attack on a domestic military installation in U.S. history. Thirteen people were killed and more than 30 were wounded.
Until an all-clear siren sounded hours after Wednesday's shooting began, relatives of soldiers waited anxiously for news about their loved ones.
"The last two hours have been the most nerve-wracking I've ever felt," said Tayra DeHart, 33, who had earlier heard from her husband that he was safe but was waiting to hear from him again.
Brooke Conover, whose husband was on base at the time of the shooting, said she found out about it while checking Facebook. She immediately called her husband, Staff Sgt. Sean Conover.
"I just want him to come home," she said.
President Barack Obama vowed a complete investigation. In a hastily arranged statement while in Chicago, Obama reflected on the sacrifices Fort Hood troops have made — including enduring multiple tours to Iraq and Afghanistan.