A lone gunman killed three people and wounded 16 others on Wednesday at Fort Lee, a Texas military base, before taking his own life. Authorities have said he was an Iraq vet with a history of mental health issues.
"Obviously, we are digging deep into his background, any criminal or psychiatric history, his experiences in combat. All of the things you would expect us to do are being done right now," said Lt. Gen. Mark Milley, the post's commanding general, CNN reported.
The shooter, special Ivan Lopez, is said to have gone from one building at Fort Hood to another, killing three and wounding 16 others with a .45-caliber handgun, before ending his own life with the weapon. Authorities initially did not rule out the possibility of terrorism, but downplayed the possibility.
Milley also addressed reports that the incident might have been the result of an argument in one of the unit areas, saying that so far officials have not found indication that that was the case. It is also not yet known whether Lopez knew the victims or not.
"My reaction was not 'not again here,'" Milley said. "My reaction was to immediately make sure we had a read on the casualties. Immediately secure the site. Immediately look for one or more shooters."
The attack is reminiscent for many of the Nov. 5, 2009, mass shooting at Fort Hood, where Maj. Nidal Hasan, who served as a psychiatrist in the army, killed 13 people and wounded over 30. He was arrested after he was shot by police, leaving him paralyzed from the waist down.
Lopez's neighbors said that he had moved to a new apartment more than a week before the shooting, with his wife and daughter, noting that the family appeared friendly and would wave and smile at people.
According to Milley, Lopez served four months in Iraq in 2011, and was undergoing diagnosis procedures for post-traumatic stress disorder. Though he was not diagnosed with PTSD, the vet suffered from other problems, including depression, anxiety and psychiatric complaints, for which he was receiving treatment and medication.
The lieutenant general added that the rules of the base prohibit people from walking around with guns, which are required to be stored in an armory. Lopez also had not registered the .45-caliber handgun he carried with him.
National Guard spokeswoman Ruth Diaz said on Thursday that Lopez was reportedly part of the National Guard in Puerto Rico at one point, but decided to join the U.S. Army instead.
President Barack Obama said in a statement Wednesday night that his administration is "heartbroken that something like this might have happened again."
"And I don't want to comment on the facts until I know exactly what has happened, but for now, I would just hope that everybody across the country is keeping the families and the community at Fort Hood in our thoughts and in our prayers," Obama stated.
"We don't yet know what happened tonight, but obviously that sense of safety has been broken once again. And we're going to have to find out exactly what happened."