In the days since a gunman wielding an AR-15 style weapon opened fire at a Texas outlet mall -- committing the second-deadliest US mass shooting this year -- witnesses have recounted the scene's horror and bloodshed, which forced several untrained bystanders to become first responders for the eight killed and several others wounded.

The shooter, identified by authorities as Mauricio Garcia -- began firing Saturday afternoon in the parking lot of the Allen Premium Outlets, investigators say. As he made his way through the complex, the sound of gunfire sent shoppers and employees fleeing for cover until the gunman was finally shot and killed by a police officer.

Though investigators have yet to pin down a motive in the attack, they are probing whether Garcia was driven by extremist ideologies, a law enforcement source told CNN. In social media posts reviewed by CNN, the gunman appears to have obsessed over Nazism, weapons and prior mass shootings.

Mall store employee Andria Gaither said she was grabbing some cardigans from the back of the store when two young girls suddenly rushed to crawl underneath a fitting room door and behind a clothing rack.

The girl behind the clothing rack was "frantically trying to open the (dressing room) door and asked me, 'Open the door! They're shooting! They're shooting!'"

When she and others were able to escape out of an emergency exit, the sound of gunshots was "very close, very loud, nonstop."

"It sounded like a very powerful gun," said Gaither. "Every gunshot sounded like an explosion."

Steven Spainhouer, an Army veteran who rushed to the scene because his son works in the mall, described the wounds he witnessed as "like in a combat zone."

He first ran to assist a young girl lying in the bushes in the parking lot, he said. But when he pulled her head back to check for a pulse, he saw that "a bright shining face was gone. There was nothing left. She was already gone."

Another military veteran, Joshua Barnwell, had taken shelter in a Lucky Jeans store but came out to tend to the injured as soon as he felt it was safe, he told CNN's Anderson Cooper.

In the parking lot, he was met with a horrific scene. Several victims were collapsed and bloody, including a dead child and others who had "multiple, vicious" gunshot wounds.

Barnwell went to an injured mother, who begged that he care for her wounded child. As he was performing CPR on the child, a "vile amount of blood came out" of the girl's back and he realized she was dead.

He returned to her mother, who had atrocious bullet wounds. "Please fight and be there for the family that you have," Barnwell said he told the woman.

Among those killed were two elementary school-aged sisters; three members of a Korean American family, including a child; a mall security guard; and a young engineer.

Barnwell said he wants people to "really and truly understand the depths of the depravity that occurred."

"If in the detail it upsets them, then I'm glad -- because it was a disastrous situation," he said.

Investigators digging for possible extremist motives

Details of the 33-year-old gunman's past have continued to surface as investigators dig in to whether his actions were motivated by right-wing extremism, a senior law enforcement source familiar with the investigation tells CNN.

When authorities examined Garcia's body, they found he was wearing an insignia investigators believe may be connected to extremist groups, the source said.

The insignia appears to be shown in a photo posted by an account user on the Russian social media website Odnoklassniki that a law enforcement source said investigators believe belongs to the shooter.

The user posted writings supporting Nazi ideology, shared images of his firearms and posted a photo of the mall in the weeks before the shooting.

A few weeks before the shooting, the user posted a screenshot from Google Maps showing what times of day the outlet mall was busiest.

Other posts espoused antisemitism and included photos of firearms and a man's torso with a large swastika tattoo over his heart, though it's unclear whether the man pictured is Garcia.

Law enforcement does not yet feel they have a "complete picture" of Garcia's past and are continuing to dig into his background, a law enforcement source told CNN.

Multiple victims were from same two families

Several of the shooting victims were from the same family and, according to the Texas Department of Public Safety, included children as young as 3.

Three Korean Americans -- husband Cho Kyu Song, 37, and his wife, Kang Shin Young, 35, as well as one of their children -- were killed, the Houston office of the South Korean Consulate confirmed, according to the Dallas Morning News. The child's name and age were not given, but the Texas Department of Safety did note a three-year-old child was among those killed. That child was identified as the couple's son James Cho, according to a GoFundMe post.

Their six-year-old son William "is the only surviving (family) member of this horrific event," a GoFundMe post written by friends of the family said.

Two sisters -- Daniela and Sofia Mendoza -- "will not be forgotten" as their peers mourn their loss, the Wylie Independent School District said in a letter to parents.

Daniela was in fourth grade and her sister was in second grade, the letter said. Their mother, Ilda Mendoza, is in the hospital in critical condition.

Other victims included Christian LaCour, an outlet mall security guard, and Aishwarya Thatikonda, who was killed while visiting the mall with a friend, CNN affiliate WFAA reported.

Seven surviving victims were hospitalized as of Sunday afternoon, including three in critical condition, Allen police said.