The commander of a US army base where a soldier shot three colleagues says an "escalating argument" was the most likely cause of the attack.
Ivan Lopez wounded 16 others before killing himself at Fort Hood in Texas on Wednesday.
It is thought the Iraq war veteran may have been suffering post-traumatic stress disorder.
Investigators believe his mental condition was not the “direct precipitating factor” in the shooting, the authorities said yesterday.
The Texas base’s commander, Lieutenant General Mark Milley, offered those details a day after saying that Special Ivan Lopez’s mental condition appeared to be an underlying factor in the attack.
Yesterday Lt Gen Milley said that an “escalating argument” precipitated the assault.
He declined to discuss the cause of the argument but said investigators believe Lopez made no effort to target specific soldiers.
Lt Gen Milley would not say whether those involved were among the dead or wounded, or how many shooting victims had been a part of the argument.
“There was no premeditated targeting of an individual,” he said.
Chris Grey, a spokesman for the Army’s Criminal Investigation Command based in Quantico, Virginia, said the military has not established a “concrete motive”.
And because Lopez is dead, he added, “the possibility does exist that we may never know why the alleged shooter did what he did”.
Lopez initially began firing near a junction, then travelled to several nearby buildings, went inside and kept firing.
While driving to those locations in his vehicle, he fired indiscriminately at other soldiers, Mr Grey said.
He also confirmed for the first time that the military police officer who confronted Lopez exchanged words with him before firing a single round at him that apparently missed.
That is when the gunman shot himself.
The authorities have interviewed more than 900 people in their investigation, Mr Grey said.
Also yesterday, Lopez’s father said his son had struggled with the recent deaths of his mother and grandfather and the stress of being transferred to a new base.
Lopez’s father, who shares the same name, said his son was receiving medical treatment but was a peaceful family man and a hard worker.
“This is very painful for me,” the elder Lopez said in the statement issued from his native Puerto Rico.
He called for prayers for the dead and the 16 people who were wounded in the rampage.
“My son could not have been in his right mind,” he said. “He was not like that.”
Wednesday’s attack was the second at the base since 2009, when 13 people were killed by Army psychiatrist Nidal Hasan.
He had said he was angry about being deployed to Afghanistan and wanted to protect Islamic and Taliban leaders from US. troops.
Lopez, an Army truck driver, did a short stint in Iraq in 2011 and told medical personnel he had suffered a traumatic brain injury.
The 34-year-old was undergoing treatment for depression and anxiety while being evaluated for post-traumatic stress disorder, base officials said.
But officials said Lopez did not see any combat in Iraq and had not previously demonstrated a risk of violence.
He seemed to have a clean record that showed no ties to potential terrorists. Lopez had arrived at Fort Hood in February Fort Bliss, another Texas base near the Mexico border.
A family spokesman said on Thursday that Lopez was upset he was granted only a 24-hour leave to attend his mother’s funeral in November. That leave was then extended to two days.
Yesterday the authorities formally identified the dead as 39-year-old Daniel Ferguson, of Florida; 38-year-old Carlos Lazaney Rodriguez, of Puerto Rico; and 37-year-old Timothy Owens, of Illinois.
Six soldiers wounded in the attack remained in hospital yesterday. Ten of the 16 have been released, Lt Gen Milley said.