Preliminary information developed during the investigation indicates the suspect was in the U.S. Army in 2008 and was "removed due to mental health concerns," law enforcement sources briefed on the probe tell ABC News.

In addition to the insignia on the shooter's equipment that suggested a right-wing extremist ideology, investigators have found social media accounts connected to the suspect that reveal hundreds of postings and images, including writings with racially or ethnically motivated violent extremist rhetoric, including neo-Nazi material and material espousing the supremacy of the white race, the sources said.

None of the subject's postings analyzed to date were liked or shared by other users nor were there any public comments, according to the sources. The suspect's account did not contain any friends or associates that were publicly visible, the sources said.

Investigators have determined the suspect had no criminal history and opened fire with a semi-automatic rifle, according to the sources. The shooter was equipped with a ballistic vest, numerous magazines and additional handguns, the sources said.

-ABC News' Josh Margolin

Christian LaCour, 20, is the first victim of the shooting at the Allen Premium Outlets to be identified.

LaCour died while working as a security guard at the outlet mall, his sister said.

"He was a really sweet kid," Brianna Smith told ABC News over the phone. "I’m sad that he’s gone.”

Christian's mother, Tracye LaCour, asked for privacy, telling ABC News that they are praying for the families of the other victims.

-ABC News' Miles Cohen

The suspect in the Allen, Texas, mall mass shooting was identified Sunday as Mauricio Garcia, according to law enforcement officials briefed on the investigation.

The 33-year-old Garcia was fatally shot by a police officer who rushed to the gunfire and confronted him at the outlet mall on Saturday afternoon, police said.

The Texas Department of Public Safety, which is leading the investigation, was searching Mauricio's home and vehicle on Sunday. The agency declined to comment further on Garcia and said the motive for the shooting remains under investigation.

President Joe Biden said the assailant was dressed in tactical gear and armed with an AR-15-style assault weapon he used to gun down eight people and injure at least nine others.

At least three people wounded in a mass shooting at an Allen, Texas, outlet mall remained in critical condition on Sunday, according to police.

Four victims were taken to Medical City McKinney, a trauma center in McKinney, Texas, about seven miles from Allen. Three of the patients were in critical condition and one was listed in fair condition, according to a statement Sunday from the Allen Police Department.

Eight victims were killed in the attack and a total of nine people were injured, police said. The shooter, whose name has not been released by police, was confronted and fatally shot by a police officer.

Among the injured, one patient was transferred from Medical City McKinney to Medical City Plano and was in fair condition. Another patient was transferred to Medical City Children's Hospital in fair condition, police said.

"We are a strong and caring community and we want all of the victims and their families impacted by this tragedy to know that we will wrap our arms around you, and we are here for you," Allen Mayor Kenneth M. Fulk said in a statement released Sunday. "The City of Allen pledges to offer our complete support. We know you are grieving, we are grieving. Rest assured, the nation and the world are also grieving."