Security officials at the nation’s largest nuclear power plant detained a contract worker with a small explosive device in the back of his pickup truck.

The worker was stopped and detained at the entrance of the Palo Verde Nuclear Generating Station. Officials then put the entire plant on lock-down barring anyone from entering or exiting the facility.

Authorities described the device as a small capped pipe that contained suspicious residue. The Maricopa County Sheriff’s Office rendered the device safe and investigators began interviewing the worker.