A gunman opened fire Saturday in a Walmart and around a nearby shopping mall in El Paso, Texas, leaving 20 people dead and 26 injured, law enforcement officials said.

In delivering an updated number of deceased, Gov. Greg Abbott called it "one of the most deadly days in the history of Texas."

"Twenty innocent people from El Paso have lost their lives," he said.

Authorities appealed for urgent blood donations for the wounded.

One suspect was in custody, officials said.

Law enforcement sources say police identified the suspect as Patrick Crusius, 21, from the Dallas area. Sgt. Robert Gomez of the El Paso Police Department said no shots were fired by law enforcement.

Multiple senior law enforcement officials believe Crusius posted a screed online just prior to the attack. They say investigators are examining a posting they suspect is from him but that they have not officially confirmed it.

The sources said it was too soon to draw any motives from the posting.

It appears the document, with no mention of the location of the attack, was posted on an extremist website at 10:12 a.m. Saturday, El Paso time, according to law enforcement sources. Police say the first 911 call alerting them to the shooting at the Walmart was made 27 minutes later ― not enough time to act.

'Nexus ... to a hate crime'

El Paso police Chief Gregory K. Allen said at an evening news conference that the crime appears to have "a nexus at this point in time to a hate crime."

However, the FBI special agent in charge of the El Paso office, Emmerson Buie, said it was too early to determine if this was indeed a hate crime and that the investigation into a motive continued.

"We're reviewing all the evidence," Buie said.

Allen said the shooting was reported at 10:39 a.m., and police were on the scene six minutes later.

Authorities initially said a second person was taken into custody, but later said they believe only one person was responsible.

The victims included at least three Mexican citizens, according to Mexican President Andrés Manuel López Obrador, who announced the figure in a video tweet.

"I regret the events in El Paso, Texas, neighboring town and brother of Ciudad Juarez and our nation," he said. "I send my condolences to the families of the victims, both American and Mexican."

What we know so far:

The shooting took place in a Walmart and around the nearby Cielo Vista Mall in El Paso.

Police received their first calls on the incident at 10:39 a.m.

20 people were killed and 26 injured.

Police put out an urgent call for blood donations in light of the high number of injuries.

One male suspect is in custody.

Close to the Mexican border, the retail area around the Cielo Vista Mall is a popular shopping destination for people on both sides of the dividing line, as evidenced by the Texas and Mexican license plates in the parking lots, although the crackdown at the border has cut down on some of that consumer traffic.

In several tweets, police initially urged people to stay away from the area, but by 1 p.m. said there was no imminent threat.

'Shocked and scared'

A Walmart employee told NBC affiliate KTSM in El Paso that she was working by the self-checkout when gunshots rang out. The employee, who only wanted to be identified by her first name, Leslie, said she initially thought boxes had been dropped.

“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,” the Walmart worker said.

Adriana Quezada, 39, was in the store with her two children when the shooting began.

"I heard the shots but I thought they were hits, like roof construction,"she told The Associated Press.

In a tweet Saturday afternoon, Walmart said it was "in shock."

Customs and Border Patrol police walk past individuals that were evacuated from Cielo Vista Mall and a Wal-Mart where a shooting occurred in El Paso, Texas, on Aug. 3, 2019. Joel Angel Juarez / AFP - Getty Images

'Blood needed urgently'

"We’re praying for the victims, the community & our associates, as well as the first responders," a statement posted on Twitter read. "We’re working closely with law enforcement & will update as appropriate."

Authorities on Saturday asked for those who want to help to consider going to area blood donation centers.

"Blood needed urgently," El Paso police tweeted. "Multiple injured transported to various hospitals."

A University Medical Center of El Paso spokesman said the victims had been taken to different hospitals. The University Medical Center received 13 victims, many with "level one" injuries, which is the most serious level, spokesman Ryan Mielke said.

He said two children, ages 2 and 9,were taken to El Paso Children's Hospital and their conditions were stabilized.

At another hospital, Del Sol Medical Center, an official said 11 victims ranging in age from 35 to 82 years old had been transported there.

President Donald Trump tweeted twice Saturday about the attack. In the latest he said, "There are no reasons or excuses that will ever justify killing innocent people."

The city's mayor and other public officials also responded.

"Our hearts go out to those who have been injured and the families of those who may have been killed," El Paso Mayor Dee Margo said during an interview with KTSM.

Rep. Veronica Escobar, D-Texas, was an hour into a town hall meeting at Coronado High School, 15 miles from the mall, when she abruptly had to end the event because of the shooting.

“You all, I am so sorry,” she said. “There is an active shooter. We are going to need to clear the event.”

The crowd gasped in response and began to move, according to the congresswoman’s Facebook Live video.

“We’ve been asked by law enforcement to just send everybody home,” she said.

A reunification site for families of possible victims was set up at a local school.

El Paso Community College said in a tweet that all of its campuses have been evacuated "out of an abundance of caution."

Democratic presidential hopeful Beto O’Rourke of El Paso told reporters Saturday night Trump's past rhetoric demeaning Mexican immigrants bears some responsibility.

The president's stance toward immigrants from south of the border "doesn’t just offend our sensibilities …. it leads to violence."

U.S. Rep. Joaquin Castro, D-Texas, speaking as chairman of the Congressional Hispanic Caucus, condemned Trump's rhetoric as a contributor to such violence.

"The language in the shooter’s manifesto is consistent with President Donald Trump’s description of Hispanic immigrants as 'invaders,'" he said in a statement. "Today's shooting is a stark reminder of the dangers of such rhetoric."

Abbott, however, said it was too soon to discuss blame and gun policy.

"There are bodies that haven’t even been recovered," he told reporters at a separate press conference. "We need to focus more on memorials before we start the politics."

Residents participated in a prayer and vigil Saturday night at El Paso High School, according to KTSM.

The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives responded to the scene to assist the El Paso Police Department.