Phoenix, Arizona – Security officials at the nation’s largest nuclear power plant detained a contract worker who had a small explosive device in the back of his pickup truck Friday, the Associated Press reported.

The worker was stopped and detained at the entrance of the Palo Verde Nuclear Generating Station, and security officials then put the nuclear station on lockdown, prohibiting anyone from entering or leaving the facility.

The explosive device was described as a small capped pipe that contained suspicious residue. Chagolla said deputies have rendered the device safe and that investigators were interviewing the worker.