Investigators are searching for answers in a shooting that killed three people and wounded 16 at Fort Hood, Texas, before the soldier suspected in the shooting killed himself.
Authorities are probing whether the gunman's four months of service in Iraq in 2011 may have contributed to his actions, but have not determined a motive.
An investigator says authorities would begin by speaking with the man's wife, and expected to search his home and any computers he owned.
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.
He said investigators have "no indication" the shooter was involved with extremist organizations.
The military has not released the shooter's name, but civilian authorities identify him as Ivan Lopez, a soldier.
Hospital officials in Texas say three of the wounded are in critical condition and two of them will require further surgery.
Base commander Lieutenant General Mark 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."
Milley said authorities do not believe the incident was related to terrorism. He said the man was in the process of being tested for post-traumatic stress disorder.
"He was undergoing behavioral health and psychiatric treatment for depression, and anxiety and a variety of other psychological and psychiatric issues."
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 spree in 2009. Thirteen people were killed and more than 30 others wounded when an Army psychiatrist opened fire on personnel.
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.
Defense Secretary Chuck Hagel called Wednesday's shooting a "terrible tragedy" for a community that has too recently see that kind of "senseless violence."