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DENVER — The Denver Police Department is increasing its vigilance as officers patrol box stores after a shooting Saturday left 20 dead and more wounded in an El Paso shopping area.

Among the possibilities being investigated is whether it was a hate crime, the El Paso police chief said. Two law enforcement officials who spoke to The Associated Press on condition of anonymity identified the suspect taken into custody as 21-year-old Patrick Crusius. El Paso police haven’t released his name, but confirmed the gunman is from Allen near Dallas.

Police said another 26 people were injured and most were being treated at hospitals. Most of the victims were believed to have been shot at a Walmart near the Cielo Vista Mall, they said, adding that 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 described many of those hurt as having life-threatening injuries. He also said police found a post online that may have been written by the suspect — one reason authorities are looking at whether it was a hate crime.

El Paso, which has about 680,000 residents, is in West Texas and sits across the border from Juarez, Mexico.

Carlos Montoya, a spokesperson for the Denver Police Department, said officers are paying extra attention to box stores in the wake of the shooting.

“We are asking our officers to be more vigilant in their patrols around the big stores—Target, Home Depot, Lowe’s—all them that might be in their area,” he said.

Although there have been no reports of a threat of a similar incident in Colorado, Montoya said the department takes any possible threat seriously, and asks anyone who sees anything suspicious to report it to law enforcement.

“When you do call in, to make sure that you can be as detailed as possible about the person you’re calling in or the vehicle you’re calling in—so that way, our officers are safe when they’re approaching that person or vehicle,” he said.

Montoya said shootings like the one in El Paso can bring up memories for Coloradans.

“When a school shooting happens, you always have memories of Columbine,” he said.  “When we have mass shootings, or big shootings like this, it brings up memories we don’t want to rehappen, so we do our best.”