An employee of the Palo Verde nuclear power plant, the larges in the U.S., was detained at the control gate of the site after the guards found a pipe bomb his vehicle, officials said.

The shocking news led to a total lock-down of the plant which is located about 50 miles (80 km) west of Phoenix, Arizona. The apartment of the suspect was searched while he was in police custody – not under arrest – and he offered his full cooperation to the officers investigating the case, the Maricopa County Sheriff's Office said.

The employee had stopped for a routine security sweep at a nuclear power plant checkpoint at about 6 a.m. when the item identified later as a pipe bomb was found in his pick-up truck, said Jim McDonald, spokesman for Arizona Public Service Company, which owns the plant.

The lock-down of the Palo Verde nuclear power plant was lifted on Friday and according to the Arizona Public Service, the operations weren’t affected.

"Our examination and preliminary testing shows it is a viable improvised explosive device," said Capt. Paul Chagolla.

The explosives experts working for the police said that the pipe bomb wasn’t powerful enough to inflict serious damage on the nuclear plant, instead it would have only damaged the car, Sheriff Joe Arpaio said.

The contract employee of the plan, identified as 61-year-old Roger Hurd of South Carolina, said that he wasn’t aware of the presence of the pipe bomb in his automobile.

The investigation team is clueless about how the pipe bomb got in the pick-up truck without Hurd knowing about it. The search of the suspect’s apartment in Phoenix didn’t provide the investigators with any leads.

"There was nothing there that would connect him to the pipe bomb," Arpaio told Reuters.

The Palo Verde Nuclear Generating Station, a nuclear power plant located in Wintersburg, Arizona, about 45 miles (80 km) west of Phoenix, is the largest nuclear power plant in the U.S. It produces over 30,000 gigawatt hours of electricity per year an amount that serves approximately 4 million people.

The strategic importance of the Palo Verde nuclear plant was so high that it and Phoenix were documented by the former Soviet Union as target locations in the event of nuclear conflict during the Cold War.

When the war in Iraq was launched in March 2003, the National Guard troops were sent to protect the plant amidst fear of a terrorist attack.

The main owner of the Palo Verde power plant is the Arizona Public Service. The other owners are Salt River Project, El Paso Electric Co., PNM Resources, Southern California Edison, Public Service Co. of New Mexico, Southern California Public Power Authority, and the Los Angeles Dept. of Water & Power.