"We have to find all those witnesses, the witnesses to every one of those shootings, and find out what his actions were, and what was said to the victims," said a federal law enforcement official who spoke on condition of anonymity, because he was not authorized to discuss the case by name.
The official said authorities would begin by speaking with the man's wife, and expected to search his home and any computers he owned.
The shooter was identified as Ivan Lopez by Texas Rep. Michael McCaul, chairman of the House Homeland Security Committee. But the congressman offered no other details, and the military declined to identify the gunman until his family members had been notified.
Lopez apparently walked into a building Wednesday afternoon and began firing a .45-caliber semi-automatic pistol. He then got into a vehicle and continued firing before entering another building, but he was eventually confronted by military police in a parking lot, according to Milley, senior officer on the base.
As he came within 20 feet of an officer, the gunman put his hands up but then reached under his jacket and pulled out his gun. The officer drew her own weapon, and the suspect put his gun to his head and pulled the trigger a final time, Milley said.
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.
Lopez was from the U.S. territory of Puerto Rico and joined the island's National Guard in 1999. He went on a peace and security mission to Egypt's Sinai Peninsula in the mid-2000s, and left the National Guard in 2010 to join the U.S. Army, said Lt. Col. Ruth Diaz, spokeswoman for the Puerto Rico National Guard.
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.
"Our thoughts and prayers are with all families involved," Diaz said. "There is loss on both sides here."
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.