An APS worker found a grenade along the side of State Route 74 Tuesday morning, near milepost 25.
State Route 74, otherwise known as Carefree Highway, was closed between I-17 and Lake Pleasant Parkway while the bomb squad detonated the device.
The scene was cleared and the road reopened.
Deputy Joaquin Enriquez from the Maricopa County Sheriff's Office said the bomb squad "had no reason to believe it was not a functioning grenade."
No one was hurt, but investigators still don't know how it got there.