Source AP ©

Palo Verde Nuclear Generating Station to carry out security investigation

United States' largest nuclear power plant was closed Friday after a contract worker entered with something security was "not comfortable with".

Security officials detained the worker at the Palo Verde Nuclear Generating Station and were conducting an investigation. The incident was considered an "unusual event" - the lowest of four emergencies the plant can declare, said Jim Melfi, an inspector with the U.S. Nuclear Regulatory Commission.

Officials did not say what the worker brought to the plant. There was no threat to the public, said Palo Verde spokesman Jim McDonald.

Palo Verde, which is operated byArizona Public Service Co., is the United State's largest nuclear power plant. Located in Wintersburg about 50 miles (80.5 kilometers) west of downtown Phoenix, the plant supplies electricity to about 4 million customers in Arizona, New Mexico, Texas and California.