Texas Gov. Greg Abbott and El Paso police say 20 people have been killed and 26 others wounded in a shooting at an El Paso shopping centre.

Law enforcement officials have said a 21-year-old Dallas-area man named Patrick Crusius is in custody in the shooting. The officials were not authorized to discuss an ongoing investigation publicly and spoke to The Associated Press on condition of anonymity.

Police say most of the victims of Saturday’s attack appear to have been at a Walmart near the Cielo Vista Mall.

Most of the victims were shot at a Walmart near the Cielo Vista Mall, El Paso police Sgt. Robert Gomez said at a news conference. He said the store was packed with as many as 3,000 people during the busy back-to-school shopping season.

“This is unprecedented in El Paso,” said Gomez, who added that many of the injured had life-threatening injuries.

Residents were volunteering to give blood to the injured, while police and military members were trying to help people who were looking 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 centre and later at the school that was serving as a reunification centre.

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

“I heard the 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 Walmart 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 think there was just one shooter.

Police said by midafternoon that a suspect was in custody and the public was no longer in danger. Gomez said the suspect, who used a rifle, was arrested without incident. Police believe he was the “sole shooter” but are continuing to investigate reports that others were involved.

The mass shooting in El Paso 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 12 people were brought to the hospital with injuries, including one that died. Two of the injured were children who were being transferred to El Paso Children’s Hospital, he said. He declined to provide additional details on the victims.

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

Texas Gov. Greg Abbott 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.