After a gunman opened fire at the Allen Premium Outlets, Joshua W. Barnwell, an eyewitness with military experience, rushed to help the wounded.

A woman who was conscious asked him to help her daughter, he said.

"This woman had massive trauma, five to six gunshot wounds," Barnwell told "Good Morning America" on Monday.

He went to the daughter to start chest compressions, he said.

"When I saw the massive amount of blood come out from her when I gave her chest compressions to her back," Barnwell said. "I knew she was gone."

Vice President Kamala Harris released a statement Sunday about the shooting, also urging Congress to act.

"Doug and I mourn for the eight adults and children who lost their lives, pray for those wounded, and send our gratitude to the first responders who ran toward danger," she said in a statement.

"All Americans deserve to be safe from gun violence. But they are not," Harris continued. "Not because we do not know the solutions. Not because the American people are divided on this issue -- even a majority of gun owners support sensible reforms."

Harris urged Congress to send a bill to President Joe Biden that would ban assault weapons and high-capacity magazines, and would introduce universal background checks.

"We need action," Harris added.

-ABC News' Molly Nagle

The FBI has provided details to the public on how they can retrieve their vehicles from the outlet mall.

Those with vehicles that were left at the mall can meet at the Allen Senior Recreation Center beginning at 5:45 p.m. local time Sunday, where they will be sent to the outlet mall, the FBI said Sunday in a press release.

The senior center will be managing the retrievals until 9:30 p.m. local time, the FBI said.

"Please drop off only one individual for vehicle retrieval," the FBI said. "Be sure to have your car keys with you."

Officials have urged the public not to go directly to Allen Premium Outlets.

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