The device was left in a box marked "highly explosive" outside a fire station on Tennyson Street in Denver on Thursday night.

Police evacuated the fire station and two nearby houses while investigators x-rayed the box. That's what one of the fire officials recognized the device.

It's a fire grenade that is thrown into a fire, detonates and helps extinguish the flames.

Investigators are now trying to determine who put the item outside the fire station. They think it may have been someone giving an antique to the fire station, but they did it in an improper fashion.