El Paso shooting: Twenty killed after gunman opens fire in Walmart

Twenty people have been killed in the latest mass shooting to take place in the US.

El Paso police say one suspect is in custody after reportedly opening fire at a Walmart in the Texas city. It was earlier reported three had been arrested at the nearby Cielo Vista Mall. The mall later said no shots had been fired on its property.

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

Walmart said on Twitter account it was "in shock".

"We're praying for the victims, the community & our associates, as well as the first responders," the retailer wrote. "We're working closely with law enforcement & will update as appropriate."

The arrested suspect is a 21-year-old man, Texas Lt Gov Dan Patrick told Fox News. CNN reported his name is Patrick Crusius. 

A manifesto expressing support for the Christchurch gunman was reportedly uploaded to the internet immediately before the El Paso attack. Its authenticity is yet to be confirmed. CNN reported the FBI is investigating it. 

Local police confirmed to El Paso NBC affiliate KTSM an image taken from CCTV footage showing a young man carrying an assault rifle was genuine.

US President Donald Trump called the incident "terrible".