Among the possibilities being investigated is whether it was a hate crime, the police chief said. Two law enforcement officials who spoke to The Associated Press on condition of anonymity identified the suspect taken into custody as 21-year-old Patrick Crusius. El Paso police haven't released his name, but confirmed the gunman is from Allen near Dallas.

Police said another 26 people were injured and most were being treated at hospitals. Most of the victims were believed to have been shot at a Walmart near the Cielo Vista Mall, they said, adding that the store was packed with as many as 3,000 people during the busy back-to-school shopping season.

"The scene was a horrific one," said El Paso Police Chief Greg Allen, who described many of those hurt as having life-threatening injuries. He also said police found a post online that may have been written by the suspect — one reason authorities are looking at whether it was a hate crime.

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

Residents were quick to volunteer 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 shots."

"But I thought they were hits, like roof construction," she said.

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 was the "sole shooter" but were continuing to investigate reports that others were involved.

El Paso police Sgt. Robert Gomez said the suspect, who used a rifle, was arrested without incident.

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.

Ryan Mielke, a spokesman for University Medical Center of El Paso, said 13 people were brought to the hospital with injuries after the Texas shooting, 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.

Gov. Greg Abbott, who confirmed the number of victims at a news conference, called the shooting "a heinous and senseless act of violence" and said the state had deployed a number of law enforcement officers to the city. President Donald Trump tweeted: "Reports are very bad, many killed."

Presidential candidate and former Texas congressman Beto O'Rourke appeared a bit shaken as he appeared at a candidate forum Saturday in Las Vegas shortly 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."

"We have to find some reason for optimism and hope or else we consign ourselves to a future where nearly 40,000 people a year will lose their lives to gun violence and I cannot accept that," O'Rourke said.

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 Saturday, 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 Saturday's — have been committed by men who are 21 or younger.