Jefferson County Sheriff's Office spokeswoman Jacki Kelley said the discovery of the bomb at the Southwest Plaza mall -- after a fire was reported near the food court on the anniversary of the deadly shooting spree at Columbine High School in Littleton -- was being kept in mind but was not a focus of the investigation, The Denver Post reported.
No arrests had been made.
Columbine High had been closed Wednesday to observe the anniversary of the day when two heavily armed teens killed 12 students and a teacher and injured 24 other students before killing themselves April 20, 1999.
The FBI told KUSA-TV, Denver, it was investigating the pipe bomb incident as a case of domestic terrorism.
Kelley said mall workers found the propane tanks while investigating smoke in a hallway shortly before noon. Police then discovered the pipe bomb nearby.
About 300 mall employees and an estimated 6,000-10,000 shoppers were evacuated into the parking lot, the Post said.
"If it's kids, they're crazy not to know what this means in this town," Margie Hecht, who lives near the mall, told the newspaper. "I remember that day, and they wouldn't think this is funny if they did."
Candy Davis, who works at the mall, said: "Nobody who works here deserves to get killed. It's crazy."
A spokeswoman for Jefferson County schools said about 25 area schools were locked down for a time Wednesday afternoon as a precaution.