Base commander Lieutenant General Mark Milley said Thursday there was a strong possibility the gunman had been in an altercation with another soldier or soldiers prior to the shooting, but says there is no indication that he was targeting anyone specifically.
Milley identified the shooter as 34-year-old Ivan Lopez, a soldier originally from Puerto Rico.
He says Lopez had a medical history that indicated unstable psychiatric or psychological conditions, which may have been an underlying cause for the attack, but adds that investigators are not ruling out any possible causes.
Authorities are also probing whether the gunman suffered from post traumatic stress disorder following his four months of service in Iraq in 2011.
U.S. Army Secretary John McHugh told a U.S. Senate committee Thursday the gunman was not directly involved in combat in Iraq. He added the shooter was seen by a psychiatrist last month and showed no signs of violence.
Defense Secretary Chuck Hagel called Wednesday's shooting a "terrible tragedy" for a community that has too recently seen that kind of "senseless violence." He said the Department of Defense will doing everything possible to implement reforms to ensure the security of the men and women working in the U.S. armed forces.
"We recognize the imperfections. We recognize the risks, everyday risks, in the jobs these men and women do for our country. But their is no mistaking the fact that the prioritization among our service leaders, our commanders, our leaders, is the safety of those men and women," said Hagel.
Hospital officials in Texas say three of the wounded are in critical condition and two of them will require further surgery. Four have been discharged.
Milley says the gunman shot himself with a semi-automatic pistol as he was approached by a military police officer.
“The exact sequence of events and timeline of events are not 100 percent clear. It is believed that he walked into one of the unit buildings, opened fire, got into a vehicle, fired from a vehicle, got out of the vehicle, walked into another building and opened fire again, and then was engaged by local law enforcement here at Fort Hood," said Lieutenant General Mark Milley.
President Barack Obama, speaking from Chicago, offered condolences. He referred to the wounded and their families as those "who have sacrificed so much for freedom."
Fort Hood was the scene of a mass shooting in 2009. Thirteen people were killed and more than 30 others wounded when an Army psychiatrist opened fire on fellow soldiers.
After that shooting, the Pentagon ordered tightened security at all U.S. bases. On Thursday, Army Chief of Staff Raymond Odierno told lawmakers he believes measures put in place after the previous Fort Hood shooting aided investigators coping with Wednesday's incident.