Security officials at the nation's largest nuclear power plant detained a contract worker with a small, crude explosive device in the back of his pickup truck Friday, and investigators were searching his apartment, authorities said.

It didn't appear to be an act of terrorism, authorities said, but they were still trying to determine why the device was in the truck.

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The worker was stopped at the entrance of the Palo Verde Nuclear Generating Station, about half a mile from the containment domes where the plant's nuclear material is stored, plant spokesman Jim McDonald said.

Security officials put the nuclear station on lockdown, prohibiting anyone from entering or leaving the facility. The lockdown was lifted a few hours later.

Authorities described the device as a six-inch capped explosive made of galvanized pipe that contained suspicious residue. Tom Mangan, a spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives, said it was likely homemade.

"If this thing went off in the bed of the truck, it certainly would put a hole in it," Mangan said. "It was rather crude in construction, but it could certainly injure somebody."

Maricopa County Sheriff's Capt. Paul Chagolla said the pipe was not hidden in the truck. He said the worker normally drove a motorcycle to work but was in a truck Friday because of cool weather.

The man, whose identity was not released, was being interviewed and authorities were searching his apartment in Phoenix with his consent, but he but had not been arrested, Chagolla said.

"There's no information to indicate that there's domestic terrorism at hand," he said.

In Washington, the Department of Homeland Security also said there was no known terrorism link.

Sheriff's officials rendered the device safe, Chagolla said.

McDonald said the worker was a procurement engineer, responsible for evaluating equipment purchases for the plant. He wouldn't say which company employed the man, whom Chagolla described as about 60 years old and originally from South Carolina.

The worker had access to some protected areas of the plant, but not the reactor areas, McDonald said.

"Our security personnel acted cautiously and appropriately, demonstrating that our security process and procedures work as designed," Randy Edington, the chief nuclear officer for plant operator Arizona Public Service Co., said in a news release.

The detention 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.

There was no threat to the public, McDonald said.

Doug Walters, the senior director of security for the Nuclear Energy Institute, an industry group, said Palo Verde's response was "exactly what you would expect it to be."

"We have a checkpoint for this reason," he said. "They were able to identify a suspicious item in the truck. I don't know what they could have done differently."

Everyone who has access to the plant must submit to a background check, McDonald said.

Workers must pass through two security checkpoints to get inside one of the plant's three containment domes, which house the radioactive material. One of the checkpoints includes an automated system that examines workers for the presence of bomb-making materials, McDonald said.

Palo Verde is the nation's largest nuclear power plant both in size and capacity. Located in Wintersburg about 50 miles west of downtown Phoenix, the plant supplies electricity to about 4 million customers in Arizona, New Mexico, Texas and California.