Fort Hood shooting: Row with fellow soldier may have motivated Ivan Lopez's fatal rampage at Texas Army base

The 34-year-old gunman had been under ongoing assessment for psychiatric issues after serving in Iraq in 2011

One day after a US Army specialist killed three people and then himself at Fort Hood, Texas, the Pentagon was trying to understand what may have motivated the fatal rampage – the second on the military base in five years – and whether it has failed again properly to protect its installations from armed insiders with deadly intent.

Investigators appeared to be focusing first on the mental health of the gunman, 34-year-old Ivan Lopez, who lived off-site with his family but entered the base with a concealed weapon. Officials saw no early evidence to tie terrorism to the incident which also left 16 wounded, three of whom remain in a critical condition.

Armed with a .45 calibre handgun, which Lopez reportedly purchased legally, he seemingly opened fire in a building on the base, then fled to his car from where he shot more rounds and finally entered a second building. When confronted by a female military police officer, he pulled his gun out and shot himself in the head.

Fort Hood’s commanding officer, Lieutenant-General Mark Milley, praised the unnamed female officer who had fired shots during the incident for her “heroism”.

He also praised a chaplain who “shielded and saved” a number of soldiers as Lopez opened fire.

Lt-Gen Milley said there was a “strong possibility” that there had been a “verbal altercation” between Lopez and another soldier shortly before the shootings. He added there were no signs that Lopez had targeted specific individuals.

Officials conceded that Lopez, who had served one four-month tour in Iraq in 2011 but not in a combat role, had been under ongoing assessment for psychiatric issues including depression, anxiety and difficulty sleeping.

While Lopez had self-reported a traumatic brain injury after his Iraq tour, there was no record of his ever having been wounded.

He was believed to have been prescribed medicines including the sleeping aid Ambien. Nothing had been detected to suggest a tendency towards violence or suicide, they said.

The tragedy reverberated across a country which still has painful memories of 2009 when an army psychiatrist, Major Nidal Malik Hasan, opened fire in a pre-deployment centre killing 13 fellow service members. Convicted and sentenced to death at a trial on the base last year, he admitted to having committed a premeditated jihad against US soldiers bound for Afghanistan.

The Pentagon promised sweeping reforms for security at military installations  in the wake of the Hasan shooting and again after the rampage last September at the Navy Yards in Washington DC which left 12 people dead. Measures were to include a greater focus on monitoring the mental condition of service personnel, particularly those returning from combat zones, and tougher background checks on contract workers with access to bases.

The latest incident marks the third time in the past six months that US military bases have been struck by deadly violence. Last month a civilian fatally shot a sailor aboard a US Navy ship at a base in Virginia.

Frustration with the limits of what can be done will be highlighted by this latest tragedy. “Nearly five years after the Nidal Hasan shooting, it is clear that we must do far more to ensure our troops are safe when they are at home on base,” the Republican Congressman Thomas Rooney said.

With Lopez, investigators seemingly have few early clues beyond his psychiatric struggles. “He had a clean record in terms of his behaviour – no outstanding bad marks that we are yet aware of,” the Army Secretary John McHugh testified before Congress.

Mr McHugh confirmed that the killer’s record showed “no involvement with extremist organisations”. Investigators were interviewing witnesses to ascertain whether an argument between Lopez and others on the base might have prompted the shootings.