One person is in custody after the shooter went on a rampage on Saturday at the Cielo Vista Mall.

Police Sergeant Enrique Carrillo told reporters that there were "multiple fatalities" and that officers were "searching multiple scenes, primarily the Walmart".

He did not say how many people were wounded or killed.

Sgt. Carrillo said a suspect was in custody and there was no longer danger to the public.

Police earlier urged people to stay away from the complex after receiving reports of multiple shooters at the mall.

Many people caught up in the attack were buying back-to-school supplies.

El Paso police Sergeant Robert Gomez said authorities had ruled out multiple shooters.

He said there were initial reports that the gunman used a rifle.

"This thing is fluid and dynamic. As of right now we don't think there's multiple (suspects) but that could change," Sgt. Gomez told reporters at the scene.

The White House said President Donald Trump has been briefed on the shooting and has spoken to Attorney General William Barr and Gov. Greg Abbott.

Gov. Abbott described the shooting as a "heinous and senseless act of violence".

"Our hearts go out to the victims of this horrific shooting and to the entire community in this time of loss", he said in a statement.

"We have deployed troopers, special agents, Texas Rangers, tactical teams, and aircraft to the scene in a support role.

"The state of Texas will do everything it can to ensure justice is delivered to the perpetrators of this heinous act."

Walmart said in a statement: "We're in shock over the tragic events at Cielo Vista Mall... We're praying for the victims, the community & our associates, as well as the first responders."

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