The FBI is investigating reports the El Paso shooting was inspired by the Christchurch massacre.
Twenty people were killed when a man opened fire in a Walmart in the large Texas city on Saturday (local time). Local officials told media at least 24 were also injured in the attack.
"Texas grieves for the people of El Paso today," Governor Greg Abbott said.
A 21-year-old white man is in custody.
Various media outlets are reporting that a manifesto was posted to online forum 8chan before the shooting, which expresses racist anti-immigrant rhetoric. The author professes their support for the alleged Christchurch gunman, who posted his own manifesto in the same forum before the March 15 massacre.
The FBI is reportedly examining the manifesto and investigating claims the Walmart shooting may have been a copycat crime inspired by the Christchurch attack.
The author of the document said they expected to die during the attack. When officers took the 21-year-old suspect into custody they used "little to minimum force", a police spokesperson said. Police did not fire their weapons once during the arrest.
Multiple American politicians have voiced their sympathy for the El Paso community on social media, including President Donald Trump. Candidates for the Democratic nomination, including Bernie Sanders and Elizabeth Warren, have condemned the nation's gun violence statistics and say action must be taken.
The El Paso attack comes less than a week after three people were shot dead at a garlic festival in California.