The largest nuclear power plant in the US has been put into "lockdown" after a suspicious device was found in the lorry of a worker, officials said.
Palo Verde supplies power to four million customers across four states
The incident occurred at the Palo Verde site in Wintersburg, about 30 miles (48km) west of Phoenix in Arizona.
The contract worker has been detained and is being questioned.
Nuclear officials say a "capped pipe with an unusual residue" was found but that the plant is on the lowest level of alert - and operating normally.
The Nuclear Regulatory Commission classes the incident as an "unusual event" - which tend to occur every few days.
No-one is allowed to leave or enter the plant during the "lockdown".
Randy Edington, chief nuclear officer of Arizona Public Service, which operates the plant, issued a statement saying: "Our security personnel acted cautiously and appropriately, demonstrating that our security process and procedures work as designed."
The name of the company the worker is employed by has not been revealed but officials said all those with access to the site had to undergo background checks.
Palo Verde supplies power to four million customers across four states.
The plant has three reactors although two were routinely out of operation on Friday.