EL PASO, Texas >> A young gunman opened fire in an El Paso, Texas, shopping area packed with as many as 3,000 people during the busy back-to-school season today, leaving 20 dead and more than two dozen injured.

Gov. Greg Abbott called the incident in the Texas border city “one of the most deadly days in the history of Texas.” Police said authorities were investigating if it was a hate crime.

The suspect was arrested without incident outside the Walmart near the Cielo Vista Mall, said El Paso Police Chief Greg Allen. Two law enforcement officials who spoke to The Associated Press on condition of anonymity identified the suspect as 21-year-old Patrick Crusius. El Paso police didn’t release his name at a news conference but confirmed the gunman is from Allen, near Dallas.

Many of the victims were shot at the Walmart, police said.

“The scene was a horrific one,” said Allen, adding that many of the 26 people who were hurt had life-threatening injuries.

The chief said police found a post online possibly written by the suspect.

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

The shooting came less than a week after a gunman opened fire on a California food festival. Santino William Legan, 19, killed three people and injured 13 others last Sunday at the popular Gilroy Garlic Festival, and died of a self-inflicted gunshot wound.

El Paso, which has about 680,000 residents, is in West Texas and sits across the border from Ciudad Juarez, Mexico.

Residents quickly volunteered to give blood to the injured after the shooting, and police and military members were helping people look for missing loved ones.

“It’s chaos right now,” said Austin Johnson, an Army medic at nearby Fort Bliss, who volunteered to help at the shopping center and later at a school serving as a reunification center.

Adriana Quezada, 39, said she was in the women’s clothing section of Walmart with her two children when she heard gunfire.

“But I thought they were hits, like roof construction,” she said of the shots.

Her 19-year-old daughter and 16-year-old son threw themselves to the ground, then ran out of the store through an emergency exit. They were not hurt, Quezada said.

She said she saw four men, dressed in black, moving together firing guns indiscriminately. Police later said they believed the suspect, who was armed with a rifle, was the only shooter.

Ryan Mielke, a spokesman for University Medical Center of El Paso, said 13 of the injured were brought to the hospital with injuries, including one who died. Two of the injured were children who were being transferred to El Paso Children’s Hospital, he said. He wouldn’t provide additional details on the victims.

Eleven other victims were being treated at Del Sol Medical Center, hospital spokesman Victor Guerrero said. Those victims’ ages ranged from 35 to 82, he said.

President Donald Trump tweeted: “God be with you all!”

At a candidate forum today in Las Vegas, presidential candidate and former Texas congressman Beto O’Rourke, who is from El Paso, appeared a bit shaken after news of the shooting in his hometown was reported. The Democrat said the shooting shatters “any illusion that we have that progress is inevitable” on tackling gun violence.

He said he heard early reports that the shooter might have had a military-style weapon, saying we need to “keep that (expletive) on the battlefield. Do not bring it into our communities.”

El Paso Mayor Dee Margo said he knew the shooter was not from his town.

“It’s not what we’re about,” he said at the news conference with the governor and police chief. El Paso is nearly a 10-hour drive from Allen.

In the hours after the shooting, authorities blocked streets near a home in Allen that is associated with the suspect. Officers appeared to speak briefly with a woman who answered the door of the gray stone house and later entered the residence.

El Paso has become a focal point of the immigration debate, drawing Trump in February to argue that walling off the southern border would make the U.S. safer, while city residents and O’Rourke led thousands on a protest march past the barrier of barbed wire-topped fencing and towering metal slats.

O’Rourke stressed that border walls haven’t made his hometown safer. The city’s murder rate was less than half the national average in 2005, the year before the start of its border fence. Before the wall project started, El Paso had been rated one of the three safest major U.S. cities going back to 1997.

Heidi Beirich, director of the Southern Poverty Law Center’s Intelligence Project, also said the El Paso shooting suspect wasn’t on her group’s radar screen prior to the shooting.

“We had nothing in our files on him,” Beirich wrote in an email.

The shooting is the 21st mass killing in the United States in 2019, and the fifth public mass shooting. Before today, 96 people had died in mass killings in 2019 — 26 of them in public mass shootings.

The AP/USATODAY/Northeastern University mass murder database tracks all U.S. homicides since 2006 involving four or more people killed, not including the offender, over a short period of time regardless of weapon, location, victim-offender relationship or motive. The database shows that the median age of a public mass shooter is 28, significantly lower than the median age of a person who commits a mass shooting of their family.

Since 2006, 11 mass shootings — not including today’s — have been committed by men who are 21 or younger.