According to authorities, someone tried to ignite the homemade explosive device packed full of shrapnel on Saturday evening but the bomb was a dud; it burned up but failed to explode.
"At the exterior of the building on the south side of the school there was remnants of an attempted pipe bomb detonation," Wendy Lippman, a spokeswoman for the Aurora Fire Department told a local CBS affiliate.
"There was not an actual explosion. The suspects that tried to detonate it were unsuccessful," she added.
School employees found the device Monday morning and alerted authorities who evacuated the children and closed the school for the remainder of the day.
After an extensive search by emergency officials and a bomb sniffing dog, no other explosives were found besides the unexploded pipe bomb.
Lippman told the Denver Post the suspects "had what they thought was the right pieces and parts to make it explode."
"They tried to start it on fire to detonate it and all they did was light it on fire and then it burned itself out," she said.
According to Lippman, the suspects were caught on surveillance footage, though no information about their identities has been released.
"They have very good video of the suspects," she said.
Students will be allowed to rmeturn to Mrachek Middle School on Tuesday.