The revelation came as the horror endured – and heroism shown – by victims in their final moments inside the Texas border city supermarket started to emerge.
One mother, aged just 25, was reportedly killed while shielding her two-month-old son as suspected gunman Patrick Crusius, 21, opened fire in the aisles.
Jordan Anchondo’s baby suffered broken bones – but lived – after she fell on him in an apparent bid to protect him from further bullets as she herself lay dying. She was shopping for school supplies for her other children at the time.
"When she got shot she was holding him and she fell on him, so that's why he broke some of his bones,” Ms Anchondo’s sister Leta Jamrowski told the Associated Press news agency. “He pretty much lived because she gave her life."
In another act of bravery, an off-duty military officer caught up in the incident has told how he carried children out from the supermarket as the shooting began.
Glen Oakley told MSNBC: "I see a whole bunch of kids running around without their parents, I got my bag in my hand I'm trying to pick up as many as I can and run out
"They're so anxious, they're dropping out of my hand."
Of the 20 people killed, they are reported to range in age from a two-year-old toddler to an 82-year-old.
Crusius himself was arrested after surrendering to police, who arrived at the supermarket – a popular Saturday afternoon destination for cross-border shoppers – within six minutes.
Greg Allen, El Paso police chief, said authorities were now working to confirm whether a racist, anti-immigrant manifesto posted online shortly beforehand was written by the suspect, who is said to have driven more than 1,000km from his home in Dallas to commit the atrocity.
The four-page document includes passages decrying the "Hispanic invasion" of the US, while the author also makes clear he intends to kill during an upcoming attack.
The first sentence expresses support for Christchurch mosque shooter who killed 51 people in New Zealand in March.
"Right now we have a manifesto from this individual that indicates to some degree, it has a nexus to potential hate crime," Chief Allen said.
At least two Democrat presidential candidates – Mayor Pete Buttigieg of South Bend, Indiana, and former congressman Beto O'Rourke – drew connections between the massacre and an apparent resurgence in white nationalism in the US.
"America is under attack from homegrown white nationalist terrorism," Mr Buttigieg said.
Mr O’Rourke, an El Paso native, went even harder during a press conference. When asked by a reporter if Donald Trump was to blame for a rise in white nationalism, he replied simply: “Yes.”
He added: "We've had a rise in hate crimes every single one of the last three years, during an administration where you have a president who's called Mexicans rapists and criminals. Though Mexican immigrants commit crimes at a far lower rate than those born here in the country, he has tried to make us afraid of them, to some real effect and consequence.”
On Twitter, the US president himself called the shooting "an act of cowardice", adding: “There are no reasons or excuses that will ever justify killing innocent people.”