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A pipe bomb was found in the truck of a contract worker when security staffers stopped him at an entrance to the nation's largest nuclear power plant Friday, a plant spokesman told USA TODAY.
The incident occurred at the Palo Verde Nuclear Generating Station about 50 miles west of downtown Phoenix.

"The sheriff's department has classified it as a pipe bomb," Jim McDonald, a Palo Verde spokesman, told USA TODAY. "How big and how powerful — I don't know." A utility company news release later described the object as a "small, capped pipe."

Maricopa County Sheriff's Office spokesman Doug Matteson said the worker was stopped because of a suspicious package in the vehicle. Sheriff's deputies were called, and the Associated Press reported the worker was taken into custody.

Capt. Paul Chagolla of the sheriff's office said sheriff's officials rendered the device safe and that investigators were interviewing the worker.

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Chagolla wouldn't release the man's name, but said the person taken into custody was a middle-aged adult male from South Carolina who was working at the plant as a contractor.

"At this point I don't have any information that would indicate that you have domestic terrorism at hand," Chagolla said.

More than 3,000 people are working at the plant now, including several hundred contract workers brought in to help with upgrades on one of the three nuclear reactors, which make Palo Verde the largest power plant in the United States.

Jim Melfi, an inspector with the U.S. Nuclear Regulatory Commission, said an "unusual event" had been declared at the facility. This is the lowest of four emergency categories that the plant can declare.

In a news release, Arizona Public Service Co., the owners of the plant, said security guards "detected a small, suspicious device in the bed of a contract employee's truck this morning as part of the normal security screening of all vehicles entering the site. The contractor was attempting to enter the site through the security check-point at the beginning of the normal day shift."

"Although initial checks did not detect any explosive material on the small capped pipe, the contractor was denied access to the site and detained for further investigation, and the Maricopa County Sheriff's Office was contacted. MCSO bomb squad tests later determined that the capped pipe was a credible explosive device," the news release said.

Two elementary schools and a high school in the area were locked down briefly when a plant employee notified the district, Superintendent Robin Berry said.

The Federal Bureau of Investigation has joined in the investigation, the utility said in the news release.

Palo Verde is located in Wintersburg about 50 miles west of downtown Phoenix. It supplies electricity to about 4 million customers in Arizona, New Mexico, Texas and California.