Scott Ostrem, 47, was taken into custody Thursday morning. But it's still not clear why he may have opened fire near a cash register at the Walmart Supercenter Wednesday night, killing two men and a woman.
"We believe as of right now that the shooting was random," Thornton Police Officer Victor Avila said. "There's nothing on motive as of yet."
The victims have been identified as Denver residents Pamela Marques, 52, and Victor Vasquez, 26, and 66-year-old Carlos Moreno of Thornton.
The gunman left the store with the same calm demeanor with which he entered, Avila told CNN affiliate KMGH
. The suspect fled the store in a red Mitsubishi Mirage, police said. Authorities searched for him through the night.
Shoppers inside the Walmart said pandemonium broke out after the gunfire erupted.
"You just hear people running right to the exit yelling, 'He's got a gun. He's got a gun,'" Marcus Smith told CNN affiliate KDVR
"For some reason these doors weren't opening. I was pushing on them, pushing on them. ... Finally we just keeping pushing the door and it opens up. I have my daughter in my arms and we run out of the door."
Virginia Samora, whose 15-year-old son was inside the Walmart, told KDVR her son texted her while his phone was on low battery.
"He said, 'I'm at 1% Mom, my battery is ready to die, but there's a shooter here at Walmart. If anything happens to me, I love you guys' " Samora said.
She said her son was able to get out and he also pulled his friend out through a side entrance.
No employees were injured, Walmart spokesman Ragan Dickens said. Counselors will be available to help employees.
"This has been a tragic situation. Our thoughts and prayers continue to be with the families who lost loved ones and on supporting our associates," Dickens said.
It's not clear when the store will reopen. On Thursday afternoon, employees were inside the store to help clean up, spokeswoman Tiffany Wilson said. Most were from other stores, but some had worked last night and came in even though they were asked not to, Wilson said.