At least 20 people were reported shot dead at an El Paso, Texas, shopping center, on Saturday, according to Texas Gov. Greg Abbott.
Reports said more than two dozen people were injured in the shooting.
The shooting occurred at the area of the Walmart and Cielo Vista Mall around 11 a.m.
Dee Margo, the mayor the city, said over Twitter that he is "devastated by the tragic events unfolding in our City. Our hearts go out to the victims and gratitude to our first responders. We will continue to keep you updated."
Multiple law enforcement agencies were at the scene and the suspect was in custody.
The suspect was identified as Patrick Crusius, 21, of Allen, Texas.
CNN reported, citing federal sources, that investigators were reviewing writings posted online days before the shootings that could reveal a motive.
The crime was being treated as domestic terrorism, according to the Wall Street Journal.