Joshua Prater, 18, was arrested after a housekeeper found a suspicious device while cleaning Prater's room, police say.
The woman reportedly brought the device to the Tempe (Arizona) Fire Department, which called the city's bomb squad to examine it. The device turned out to be an active improvised explosive device (IED), azfamily.com reports.
"They had it X-rayed, they saw it was a valid IED," Tempe Police Sgt. Mike Pooley said, according to MyFoxPhoenix.com. "It was something that wasn't big, but could cause serious injuries and the death of someone."
"There was fuse that was coming out from the device which would be how you light it and it would cause the fuse to go in there and explode," Pooley said. "The bomb was ready to go. It was active. All it needed was the trigger."
The bomb squad dismantled the bomb and detectives searched Prater's home, where police said they found more materials used to make explosives.
Prater was charged with misconduct involving weapons and possessing a prohibited weapon.
A police report indicates Prater admitted to building the device but said he did so around eight years ago and didn't know it was filled with explosive powder. Prater's friends told azfamily.com that the senior is a straight-A student who planned to attend the University of Arizona.