· At least 20 killed, 26 wounded 

· Suspect in custody is a 21-year-old white male

· Motive is under investigation, could be hate crime 

A gunman killed at least 20 people and wounded 26 others at a Walmart shopping center in El Paso, Texas on Saturday. Local police said they are investigating a manifesto that indicates this mass shooting could be a hate crime. 

El Paso police have ruled out multiple shooters, and initial reports showed that the gunman used a rifle. The suspect, now in custody, was identified as a white male in his 20s.

A manifesto that may be from the suspect showed to some extent "a nexus to a hate crime", said El Paso Police Chief Greg Allen during a press conference. 

The rampage took place as many parents were buying back-to-school supplies. 

"This is just a tragedy that I'm having a hard time getting my arms around," said El Paso Mayor Dee Margo.

Citing a law enforcement source, El Paso television station KTSM published on its website what it said were two photos of the suspect taken by security cameras as he entered the store.

The images showed a young white man wearing glasses, khaki trousers and a dark T-shirt, and pointing an assault-style rifle. He appears to be wearing headphones or ear defenders.

U.S. President Donald Trump said that the reports from El Paso were "very bad, many killed" on Twitter.

"Spoke to Governor to pledge total support of Federal Government," he wrote. "God be with you all!"

University Medical Center of El Paso received 13 patients, including one who died, according to hospital spokesman Ryan Mielke.

Some of the patients were in surgery while others were in stable condition, he added.

Two of the patients who arrived at the hospital were children with non-life threatening injuries. They were transferred to El Paso Children's Hospital.

Local media said there was such an overwhelming response to an appeal by the police department for blood donations to help the wounded that long lines formed at medical centers, some of which had to tell would-be donors to come back on Sunday.

Multiple law enforcement agencies raced to the scene at the Walmart and nearby Cielo Vista Mall, including police, state troopers, Homeland Security agents and border patrol.

Graphic video from the scene posted on social media showed what appeared to be dead bodies and wounded victims. 

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 and our associates, as well as the first responders."

Stores at the mall were also locked down as police officers cleared the shopping center in the east of the city, which lies on the southern U.S. border with Mexico.

Video posted on Twitter showed customers at one department store being evacuated with their hands up.

Mass shootings are common in the United States. On Sunday, a teenage gunman opened fire with an assault-style rifle on the crowd at a food festival in Northern California, killing three people before fatally shooting himself.

(With input from agencies)