Police spokesman Victor Avila said on Wednesday night that the shooting appeared random and there are no indications it was an act of terror.
Copter4 flew over the scene of Ostrem's arrest.
Thornton police spokesman Victor Avila said Thursday that 47-year-old Scott Ostrem had a minor criminal history, but Avila didn't elaborate.
The 5-hour delay to say the suspect in the shooting was armed and risky - and still on the run - was reportedly due to investigators working to gather information on who that person was, according to Thornton Police.
Three people were shot, all fatally.
After the shooting, it took five hours for law enforcement to notify the public that a suspect in the triple homicide was still on the loose and to distribute his photo.
Police had yet to determine a motive for the shooting, which occurred late Wednesday inside the busy Walmart about 10 miles (16 kilometers) north of Denver in a busy shopping center.
Two men died inside the Walmart, and a woman died later at a hospital.
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Court records also show Ostrem filed for Chapter 7 bankruptcy in 2015.
An anonymous tipster spotted Ostrem's vehicle not far from his apartment at around 8 a.m. Thursday and notified police, who Avila says were nearly immediately on his tail. Thornton Police said two men and one woman were shot and killed.
Once officers were able to get inside, they sifted through surveillance video to get a name and description of the suspect.
"The employees started screaming and the customers started screaming" as people began to flee, he said.
"It took a while because it was so many people, it was mass chaos in there", Avila said.
Police are asking anyone with information about Ostrem's whereabouts to come contact authorities.
Customers and employees hid or fled toward the exits after gunshots rang out in Thornton, which is a suburb of Denver.