The discovery came Wednesday on the 12th anniversary of the Columbine High School shooting massacre that left 12 students and one teacher dead.
Investigators are trying to determine if the incidents are linked.
"We're concerned about the date, the time, things of that nature, but we don't have anything solid that would indicate any link at all other than, certainly, circumstances," Jefferson County Sheriff Ted Mink said Thursday.
Wednesday's incident began when a small fire broke out at the Southwest Plaza Mall. A security guard quickly put it out but firefighters then found the pipe bomb and propane tanks, which did not explode.
No one was injured at the mall.
No arrests have been made, but dozens of law enforcement agents were scouring security video and following other leads to identify a man seen entering the mall through a side door not normally used by the public.
Authorities also were investigating the cause of the fire, including whether it was sparked by the failed detonation of the bomb.
The Associated Press contributed to this report.
