Federal authorities are treating the El Paso shooting as a case of domestic terrorism, according to the US Attorney for the Western District of Texas.
The Justice Department is also "seriously considering" bringing federal hate crime and federal firearm charges, which carry a possible penalty of death, against the suspect, US Attorney John Bash said in a news conference.
Three sources have identified the suspect as 21-year-old Patrick Crusius of Allen, Texas.
The case appears to meet the statutory definition of domestic terrorism, Bash said. Saturday's shooting at an El Paso shopping center -- which left at least 20 dead and 26 injured -- "appears to be designed to intimidate a civilian population, to say the least," he said.
"We're going to do what we do to terrorists in this country, which is to deliver swift and certain justice," Bash said.
El Paso County District Attorney Jaime Esparza said the suspect faces capital murder charges and will be eligible for the death penalty.
"We will seek the death penalty," he said.
Authorities are investigating a racist, anti-immigrant screed that they believe was posted by a man who opened fire shortly after 10:30 a.m. local time.
About 20 minutes earlier, a post on the online message board 8chan believed to be from the suspect laid out a dark vision of America overrun by Hispanic immigrants. The 2,300-word document, which police called a "manifesto," was attached to a post that said, "I'm probably going to die today."
The writing is filled with white nationalist language and racist hatred toward immigrants and Latinos. It blames immigrants and first-generation Americans for taking away jobs.
Although authorities are still investigating the suspect's connection to the document, Texas Gov. Greg Abbott called the shooting a "hate crime" during an evening news briefing, and the FBI has also opened a domestic terrorism investigation into the massacre, a source familiar with the investigation process said Saturday.
"This is disgusting, intolerable. It's not Texan," Abbott told reporters who asked about the document. "We are going to aggressively prosecute it both as capital murder, but also as a hate crime, which is what it appears to be, without having seen all the evidence yet."
FBI El Paso Special Agent in Charge Emmerson Buie said Saturday more investigative work was needed to determine whether there was a possible hate crime.
The mass shooting happened in one of the largest and safest cities on the US-Mexico border, a place central to the Trump administration's hardline stance on immigration and a city that Rep. Cesar Blanco called "ground zero" of the administration's family separations policy .
El Paso County had an estimated population of 840,000 people as of July 2018, according to the US Census Bureau . An estimated 83% were Hispanic or Latino.
A week bookended by shootings
The El Paso shooting was just one of several that happened across the country this week.
It was followed 13 hours later by
another mass shooting
in Dayton, Ohio.
At least nine people were killed and at least 26 were injured in a popular nightclub district downtown by a lone suspect, who has not been publicly identified by authorities. Dayton Police said the suspect is dead.
Last Sunday, a 19-year-old gunman wearing tactical gear opened fire at at the Gilroy Garlic Festival in California , killing three people, including a 6-year-old boy and a 13-year-old girl.
And gunfire erupted Tuesday at another neighborhood Walmart in Mississippi, killing two employees and wounding a police officer.
At least 34 people were left dead across all four scenes.
Walmart trip turns deadly
The first call of an active shooter went out at 10:39 a.m. local time, El Paso Police Chief Greg Allen said. The first officer arrived on scene at the Walmart near the Cielo Vista Mall six minutes later.
Multiple agencies responded to the scene, including the FBI, the sheriff's department, the state Department of Public Safety and Border Patrol.
Adria Gonzalez was walking by the meat section on a Saturday morning shopping trip with her mom when she heard the shots.
"They didn't sound like gunshots, it sounded more like something fell to the floor," Gonzalez, 37, told CNN.
She said she stepped up to see what was happening and that's when she saw the shooter near the main entrance, wearing headphones or earmuffs and carrying a gun.
"I could only think about getting my mom and others out of the store," she said. "We gotta get out of here."
After she saw the shooter, Gonzalez said she saw a grade-school aged boy get shot in the leg, a man bleeding on the floor and another man, covered in blood, reaching for a nearby child.
"We heard two to three shots, four to five shots, then six to seven shots," Gonzalez told CNN.
She and her mother took off -- along with about 40 other people, Gonzalez said. They waited in a storage area of the store until they no longer heard the gunfire.
"You see this happening in the movies, but when you live it yourself, when you see a person killing, the blood everywhere, you are in shock," she said.
'Manifesto' posted to 8chan
Saturday's shooting in El Paso is at least the third atrocity this year where a shooting suspect is thought to have posted to 8chan in advance of an attack.
Before the mass shooting in Christchurch, New Zealand , in March, an account believed to belong to the gunman posted a link to an 87-page white nationalist manifesto on Twitter and 8chan.
And 73 minutes before the deadly shooting at Congregation Chabad synagogue in Poway, California, in April, someone identifying himself as the suspect in that attack posted a link to a hate-speech-filled manifesto hyperlinked on 8chan.
On Saturday, the writer cited a fear that an influential Hispanic population in Texas would make the state a "Democratic stronghold," though he said "the Republican Party is also terrible," because the GOP is in his mind pro-corporation, which can lead to more immigration.
The writer of the document said they held these beliefs for years, before Donald Trump was elected President.
The author says it took less than a month to plan the shooting.
'This was a massacre'
Officials from two local hospitals said they had received at least 24 people.
Thirteen people were taken to University Medical Center of El Paso, spokesman Ryan Mielke told CNN, and one of them has died. Two children with non-life-threatening injuries were transferred to a children's medical facility, Mielke said.
Eleven victims were transported to the Del Sol Medical Center, Dr. Stephen Flaherty said. Eight are in stable condition and three are in critical condition, and the patients ranged in age from 35 to 82, he said.
Of the 20 killed, three were Mexican nationals, according to a tweet from President Andrés Manuel López Obrador. Of the 26 injured, six were Mexican, Foreign Affairs Secretary Marcelo Ebrard wrote on Twitter.
"The ages and genders of all these people injured and killed are numerous in the age groups," Allen said. "The situation, needless to say, is a horrific one."
"This was a massacre," US Rep. Veronica Escobar, who represents the area, told CNN. Escobar had received conflicting reports on the numbers of casualties, she said, but added, "The numbers are shocking."
The victims inside the Walmart will remain at the scene where they were gunned down. Authorities say the bodies cannot be removed until the crime scene is processed -- which could take days.
"One murder occurrence can take all day, now we have 20 victims, El Paso Police Sgt. Robert Gomez told reporters Saturday, adding that police are working to diagram the scene, collect evidence and identify victims. "It may be exponentially longer."
"I can tell you that the investigators are doing their best to complete this investigation proper with the respect and dignity that the victims deserve," Gomez said.
A video, shot from outside the Walmart, showed people lying on the ground, some of them next to a table set up by the store's entrance.
"There's a man lying down at the stand that a school set up," the man holding the camera says in Spanish.
"Help!" a man screams in English.
"We need CPR," someone else says. "We need CPR."
Walmart issued a statement regarding the shooting, saying, "We're in shock over the tragic events at Cielo Vista Mall in El Paso. ... We're praying for the victims, the community & our associates, as well as the first responders."