Lt. Gen. Mark Milley, the senior officer on base. (Tamir Kalifa/AP)

The commander of Fort Hood said that officials believe the gunman’s mental health problems were the fundamental cause of Wednesday’s shooting.

Still, they are working “to figure out what the trigger event” was before the shooting, Lt. Gen. Mark A. Milley said in a news conference Thursday afternoon.

“At this point, we have not yet ruled out anything whatsoever,” he said.

Officials have been told Lopez had an argument with someone else that seemed to precede the shooting, but that has not been confirmed, Milley said.

Milley did confirm Thursday that the shooter is Army Spec. Ivan A. Lopez, 34, a native of Puerto Rico. Lopez’s identity had been confirmed by officials, but this is the first time a Fort Hood official had used his name.

Four of the 16 people who were wounded have been released from the hospital, according to Milley.

There are no indications Lopez “was targeting specific people,” Milley said, but he adds that he will await the results of the investigation before determining that for sure.

The shooting began shortly before 4:15 p.m. local time. The first word came out via a 911 call at 4:16 p.m. made by soldiers on the base who demonstrated heroism and a strong presence of mind in making the call so quickly, Milley said.

Four minutes later, an unidentified female military police officer encountered him. After Lopez took out his weapon, a .45-caliber Smith & Wesson, she opened fire, though it’s unclear if she hit him.

After that, Lopez took his own life, Milley said. The unidentified officer was not wounded, he added.

This post was updated throughout the press conference. First published: 4:18 p.m. Last update: 4:42 p.m.