Twenty people were killed and more than two dozen injured in a shooting in a busy shopping centre in the Texas border town of El Paso, the state’s governor said.
Police said among the possibilities being investigated is whether it was a hate crime.
Two law enforcement officials identified the suspect taken into custody as 21-year-old Patrick Crusius of the Dallas area.
Police said another 26 people were injured, with most of the victims believed to have been shot at a Walmart near the Cielo Vista Mall.
The store was packed with as many as 3,000 people during the busy back-to-school shopping season.
“The scene was a horrific one,” said El Paso Police Chief Greg Allen, who added that many of the wounded had life-threatening injuries.
El Paso Mayor Dee Margo said police were investigating whether a document posted online shortly before the shooting was written by Crusius.
In it, the writer expresses concern that an influx of Hispanics into the United States will replace ageing white voters, potentially turning Texas blue in upcoming elections and swinging the White House to the Democrats.
The writer is also critical of Republicans for what he described as close ties to corporations and degradation of the environment.