A Texas mother likely died trying to protect her baby boy from gunfire as shooter Patrick Crusius unleashed one of America's worst gun massacres on back-to-school shoppers.

El Paso resident Leta Jamrowski learned today that her 25-year-old sister, Jordan Anchondo, was among the 20 people shot to death at Walmart while shopping for back-to-school supplies.

Another 26 were injured.

The 19-year-old Jamrowski said Anchondo, a mother of three, was possibly trying to shield her 2-month-old son and fell on him as she was shot.

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She said the boy suffered broken bones and is being treated at an El Paso hospital. Jamrowski said the child "pretty much lived because she gave her life."

Jamrowski paced inside a waiting room at the hospital awaiting word of both her nephew's condition and whether her brother-in-law, Andre Anchondo, survived the attack at a busy El Paso shopping area.

"They said that if he were alive, more than likely he would have gotten in contact by now," Jamrowski said.

The shooter was charged with capital murder, and US Attorney John Bash said federal investigators are treating the shooting as domestic terrorism.

Wearing earmuffs for sound protection, Crusius, who lived a nine-hour drive or more than 1000km away in Dallas, spent 20 minutes shooting on a sunny Saturday before calmly handing himself over to police "without incident".

Local media published this image of the shooter at the Walmart.
Local media published this image of the shooter at the Walmart.

He was then captured on the store's CCTV cameras, which showed him in cargo pants and black T-shirt as he raised his gun and fired at the Cielo Vista Mall, injuring at least 26 and killing 20.

The victims ranged in age from 82 years to just a four months old. Also injured were a two-year-old child and a nine-year-old. Mexican President Andrés Manuel López Obrador said three Mexican nationals were among those killed. At least nine of the injured were in critical condition.

The assault began about 11am local time, the store packed with about 1000 shoppers.

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"On a day that would have been a normal day for someone to leisurely go shopping turned into one of the most deadly days in the history of Texas," said Governor Greg Abbott.

After the shooting victims were filmed being wheeled out of the supermarket on shopping trolleys.

Witnesses who described the horror said Cruscius "started shooting everyone, aisle by aisle, with rage".

Panic was unleashed in the crowded store, with a Walmart worker named Leslie describing the scene to local TV station KTSM.

"I thought it was just like loud boxes being dropped or something, until they got closer and closer," she said.

"That's when I looked at my co-worker, and we looked at each other, like shocked and scared.

"I got all the people that I could, I even found a little girl that was missing from her parents, and I got her, too. I tried to get as many people as I could out."

A woman named Vanessa told Fox News she was walking into Walmart when she heard what she describes as "fireworks, really loud fireworks."

"You could hear the pops, one right after another and at that point as I was turning, I saw a lady, seemed she was coming out of Walmart, headed to her car," she said.

"She had her groceries in her cart and I saw her just fall."

The alleged gunman is taken away after his arrest. Photo / Twitter
The alleged gunman is taken away after his arrest. Photo / Twitter

Police said Crusius faces capital murder charges, which carry a life sentence or the death penalty.

US President Donald Trump responded to the incident on Twitter.

"Terrible shootings in El Paso, Texas. Reports are very bad, many killed … God be with you all!"

It was the 249th gun massacre this year in the US and the most deadly since the 2017 Las Vegas massacre that killed 59. It was one of many recent attacks on public spaces in the US and came amid heightening tensions ahead of next year's election.