(CNN) Twenty people were killed and more than two dozen were injured in a mass shooting at an El Paso shopping center on Saturday, according to Texas and local authorities.

"Lives were taken who should still be with us today," Gov. Greg Abbott said at a news conference near the Walmart and an adjacent shopping mall.

Twenty-six people were injured, according to El Paso Police Chief Greg Allen.

"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."

A 21-year-old white man from Allen, a suburb of Dallas about a 650-mile drive away, is in police custody, Allen said. Authorities are looking at potentially bringing capital murder charges against him.

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The case also has a "nexus to a potential hate crime," he said.

"Right now, we have a manifesto from this individual that indicates to some degree a nexus to a potential hate crime," Allen said.

FBI El Paso Special Agent in Charge Emmerson Buie said more investigative work was needed before determining whether there was a possible hate crime.

A source familiar with the investigative process said later Saturday that the FBI has opened a domestic terrorism investigation into the massacre. It will coincide with the state inquiry and Texas authorities will take the lead in the case.

The dead included three Mexican nationals, according to a tweet from President Andrés Manuel López Obrador. Six Mexicans were injured, Foreign Affairs Secretary Marcelo Ebrard wrote on Twitter.

Authorities on the scene of a shooting at a Walmart near Cielo Vista Mall in El Paso.

CNN reported the suspect is 21-year-old Patrick Crusius of Allen, according to three sources.

Two federal law enforcement sources and one state government source confirmed the suspect's identity. The federal sources said investigators are reviewing an online writing posted days before the shooting that may speak to a motive.

The online posting was believed to be written by Crusius, the sources said, but that has not been confirmed.

'This was a massacre'

The first call of an active shooter went out at 10:39 a.m. local time, Allen said. The first officer arrived on scene six minutes later.

El Paso Police Sgt. Robert Gomez previously told reporters police were initially given multiple possible locations for the shooting, at the Walmart and the Cielo Vista Mall next door.

"This is a large crime scene, a large area," Gomez said of the scene.

Multiple agencies responded to the scene, including the FBI, the sheriff's department, the state Department of Public Safety and Border Patrol.

The crime scene will "be in play for a long period," Allen said. "Unfortunately, the deceased will remain at the scene until the scene is processed properly for evidentiary purposes to be gathered for later prosecution."

El Paso Police Department Sgt. Robert Gomez briefs media on a shooting that occurred at a Walmart.

Officials from two local hospitals said they had received at least 23 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, hospital spokesman Victor Guerrero said. Nine are in critical but stable condition, he said.

At least two of the patients are in a "life-threatening predicament," according to Del Sol Medical Center Dr. Stephen Flaherty. He said the patients ranged in age from 25 to 82. Two are in stable condition, he said, and seven required emergency operations.

"This was a massacre," US Rep. Veronica Escobar, who represents the area, told CNN. Escobar has received conflicting reports on the numbers of casualties, she said, but added, "The numbers are shocking."

Footage shows people lying on the ground outside Walmart

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."

Inside the mall, crowds hid inside stores after hearing reports of an active shooter, according to 26-year-old Brandon Chavez, an employee at Forever 21.

Chavez had just started his shift when he saw customers and staff members running to the stock room to take shelter.

"There were about 20 children and adults, plus employees, hiding, all cramped like sardines," he told CNN. "Most of us were desperate, some were on their phones. There were girls crying, people trying to talk to each other and women with babies in their arms."

Shoppers exit with their hands up after a shooting in El Paso, Texas, on Saturday.

Store employees had closed the glass doors but he could see police officers walking around the mall and evacuating people from other stores.

After police officers knocked on the store's doors, Chavez said his group had to leave the store, forming a line with their hands up and running.

In a shaky Snapchat video aired by CNN, a woman holding the camera frantically runs with a small group of girls or women through a mall department store and into a parking lot.

As the group hurries past racks of clothes and cases of merchandise, voices off camera shout, "Hands up!"

Once in the parking lot, one member of the group asks, "What happened?"

"I don't know," the woman holding the camera responds. "I don't know."

Another 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.

Authorities respond to an active shooter at a Walmart near Cielo Vista Mall in El Paso.

"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."

'Our community will heal,' mayor says

Mayor Dee Margo said Saturday evening that his city would rise above this "senseless and evil act of violence."

"We will be defined by the unity and compassion we showed in the wake of this tragedy," he said. "United, our community will heal."

Nowhere was that spirit more on display than at blood donation centers. Donations were urgently needed, and authorities said if local residents wanted to help, they should make appointments.

Frances Yepez, waiting in line at one blood donation center, said the center was at max capacity and dozens of people were waiting to make appointments for Sunday or Monday.

"It's easy to make a dollar, but it's harder to make a difference," she said. "So I get out there and do whatever I can do to help."

She said the mood there was somber, and she could hear sniffling as the crowd of people learned updates over the television.

President calls shootings 'an act of cowardice'

President Donald Trump tweeted his "heartfelt thoughts and prayers."

"Today's shooting in El Paso, Texas was not only tragic, it was an act of cowardice," the President wrote. "I know that I stand with everyone in this country to condemn today's hateful act. There are no reasons or excuses that will ever justify killing innocent people."

Democratic presidential candidate Beto O'Rourke, who once represented the area in Congress, addressed the shooting while at a speaking event in Las Vegas.

"We know there is a lot of injury, a lot of suffering in El Paso right now," he said. "I am incredibly sad and it is very hard to think about this."

"But I'll tell you, El Paso is the strongest place in the world," he added. "This community is going to come together."

O'Rourke said he would be cutting short his trip to return to El Paso.

The scene was unfolding in the same week two employees were fatally shot at a Walmart store in Southaven, Mississippi, and three people were shot and killed at the Gilroy Garlic Festival in California.

Purported online writings

Investigators believe the shooting suspect posted a document on online message board 8chan.

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, blaming immigrants and first-generation Americans for taking away jobs.

The writer cites 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 Trump was elected president.

The author says it took less than a month to plan the shooting.