A gunman with neo-Nazi leanings who killed eight people at a suburban Dallas shopping centre took eight legally purchased guns to the scene, apparently chose his victims randomly and was shot dead by police within four minutes, authorities have said.

The Allen police officer who shot and killed 33-year-old Mauricio Garcia, ending Saturday’s attack, acted heroically and saved “countless lives” through his quick action, Hank Sibley, the regional director of the Texas Department of Public Safety, said at a news conference.

He said the officer is still processing what happened and is not ready to have his name made public.

“If he hadn’t have been there, I think we’d have had a much more severe situation,” Mr Sibley said.

The investigation into Garcia’s motive is continuing, but he expressed neo-Nazi beliefs, Mr Sibley said, adding that Garcia had no criminal history before he opened fire at Allen Premium Outlets.

Garcia took eight guns to the scene, including three on him and five still in his vehicle, Mr Sibley added.

“The big question that we’re dealing with right now is, ‘What’s his motive? Why did he do this?’” Sibley said. “Well, the big question is, we don’t know. That’s what the investigation is trying to find out.”

Posts by Garcia on a Russian social networking site suggest that he planned for weeks before he launched the attack in Allen, a diverse community of about 100,000 people roughly 25 miles north of central Dallas.

Garcia researched when the shopping centre was busiest — Saturday afternoons — and posted photos on social media in mid-April of a store near where he ultimately began shooting people. Among those killed were two primary school-age sisters, a couple and their three-year-old son, and a security guard.

An Associated Press review of Garcia’s online activity shows he displayed a fascination with white supremacy and mass shootings, which he described as sport. Photos he posted showed large Nazi tattoos on his arm and torso, including a swastika and the SS lightning bolt logo of Hitler’s paramilitary forces.

The online statements have contributed to an emerging picture of Garcia, who was discharged from the Army in 2008 because of mental health issues and had apparently been working as a security guard, according to neighbours and an Army official.

Aric Toler, director of training and research at the international research collective bellingcat.com, said he identified Garcia’s profile on the site OK.RU by searching for active accounts with his birthdate located in the US.

The Associated Press independently verified the account, which also featured an image of a traffic ticket with Garcia’s name and birthdate as well as paperwork from a motel where he stayed before the shooting at Allen Premium Outlets in one of Dallas’s most diverse suburbs.

Federal agents investigating what motivated the shooting have also reviewed the online posts, according to a federal law enforcement official.

The official said Garcia had a patch on his chest when police killed him that read RWDS, an acronym for “Right Wing Death Squad”, which is popular among right-wing extremists and white supremacy groups.

Investigators have also interviewed family members and associates of Garcia to ask about his ideological beliefs and are examining his financial records and other electronic media, the official said.

Garcia joined the Army in 2008 but was terminated three months later without completing his initial training, US Army spokeswoman Heather J Hagan said.

According to an Army official he was kicked out due to mental health issues.

Garcia received an “uncharacterised” discharge, which is common for recruits who do not make it through training or the first 180 days, according to a defence official. That type of discharge — which is not dishonourable — would not set off red flags or require any reports to law enforcement.

On the Dallas block where Garcia lived at a family home until recently, neighbours said they thought he worked as a security guard but they were not sure where.

A woman who lives next door said she did not know her neighbours well but described them as nice and polite. Garcia was always friendly, she said.

A law enforcement official said investigators also have searched a Dallas motel where Garcia had been staying ahead of the attack.

Amid protests on Monday at the Texas Capitol for stricter gun control, two Republicans sided with Democrats to advance a bill that would raise the age to buy semi-automatic rifles from 18 to 21, though the measure has little or no chance of becoming law.