At least thirty people were killed at two separate attacks in the Texas city of El Paso and Dayton, Ohio on Saturday.
In the first attack, twenty people have been killed and 26 injured in a mass shooting at El Paso.
The massacre happened at a Walmart store near the Cielo Vista Mall, a few miles from the US-Mexican border.
A 21-year-old man is in custody. Police said the suspect was a resident of the Dallas-area city of Allen, about 650 miles (1,046km) east of El Paso.
He has been named by US media as Patrick Crusius.
CCTV images said to be of the gunman and broadcast on US media show a man in a dark T-shirt wearing ear protectors and brandishing an assault-style rifle.
The police and FBI are investigating whether an anonymous white nationalist “manifesto”, shared on an online forum, was written by the gunman. The document says the attack was targeted at the local Hispanic community.
US President Donald Trump described the attack as “an act of cowardice”.
Today’s shooting in El Paso, Texas, was not only tragic, it was an act of cowardice. I know that I stand with everyone in this Country to condemn today’s hateful act. There are no reasons or excuses that will ever justify killing innocent people….
— Donald J. Trump (@realDonaldTrump) August 4, 2019
Hours later, nine people have been killed and at least 16 injured in a mass shooting in Dayton, Ohio
Footage posted on social media showed people running as dozens of gunshots echoed through the streets.
It is thought the shooting took place outside Ned Peppers Bar on E 5th Street.