Officials now believe that an argument was a “direct, precipitating factor” leading to Wednesday’s shooting at Fort Hood,  Lt. Gen. Mark A. Milley, the post’s commander, said during a news conference Friday afternoon.

Army Spec. Ivan A. Lopez opened fire on Wednesday, killing three people and injuring 16, leading to a frenzied, chaotic scene on and around the sprawling installation.

“His underlying medical conditions are not a direct precipitating factor,” Milley said Friday. “We believe that the immediate precipitating factor was more likely an escalating argument in his unit area.”

On Thursday, the day after the shooting, Milley had said that the gunman’s mental health was believed to be the fundamental cause of the shooting. Milley said Friday that he wasn’t saying Lopez’s mental health was not a factor, but just that it didn’t immediately cause the shootings.

Witnesses have confirmed that he had an argument just prior to opening fire, said Chris Grey, spokesman for the Army Criminal Investigation Command.

Lopez first opened fire at 4:16 p.m., according to Grey. He fired “indiscriminately” while moving across the area, leaving a crime scene the size of two city blocks, Grey said.

No “concrete motive” has been established yet, Grey said. And he added that since Lopez took his own life on Wednesday, “the possibility exists that we may never know why [Lopez] did what he did.”

Soldiers involved in the argument were victims of Wednesday’s shooting, Grey said, but he did not immediately know if any of them were among the people killed.

Ten of the 16 people injured in the shooting have been released from the hospital, Milley said.