Twenty people have been killed and 26 injured in a mass shooting in the Texas city of El Paso.
Governor Greg Abbott described it as “one of the most deadly days in the history of Texas”.
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 victims have not yet been named. However, Mexico’s President Manuel Lopez Obrador said three Mexicans had been killed and six wounded in the attack, Reuters news agency reports.
The attack came less than a week after a teenage gunman killed three people at a California food festival. The Texas shooting has been dubbed the eighth deadliest in modern US history.
“We as a state unite in support of these victims and their family members,” Mr Abbott said.
“We must do one thing today, one thing tomorrow and each and every day after this – we must unite.”
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.