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Maricopa County Sheriff Joe Arpaio says he has arrested a man working at the Palo Verde Nuclear Plant for being in the country illegally.

Thirty-year-old Cruz Loya Alvares was taken into custody Wednesday by sheriff's deputies and interrogated. Alvares allegedly admitted to being in the U.S. illegally for the better part of the past 15 years.

Alvares was deported in 2000, but paid a human smuggler for re-entry into the U.S., Arpaio says. And last month, Mesa Police cited Alvares for driving on a suspended license.

According to MCSO, Alvares tried to gain access to the nuclear power plant on Monday but was denied because his Mexican driver's license was expired.

He tried again on Tuesday, this time as a passenger in a contractor's vehicle. He presented an Arizona identification card and was allowed in.

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When plant security examined his identification card some time later, they thought it might be fraudulent and called the sheriff's office.

According to APS, Alvares had been on site over the past two weeks doing remodeling work at an administrative building.

Sheriff Arpaio says that two employees told him that drivers of contractor's vehicles can "vouch" for passengers if they don't have IDs on them.

"In post 9/11 times, 'vouching' for employees who contractors know little about is not good practice for a facility as critical as a nuclear power plant," Arpaio added.

Palo Verde is the nation's largest nuclear plant. Alvares worked for DRP Construction, who is sub-contracted by Palo Verde Nuclear Plant.

"He never had any kind of access to what we call the protected area of the plant, that's where all the nuclear reactors are located, that's where all the vital components are. He never had any access to that, the possibility of that happening is zero," says APS spokesperson Jim McDonald.

There's no evidence that Alvares posed a threat to the nuclear plant, but the fact he was able to slip in is raising serious security questions.

"We will take a close look at the episode, a very close look, and if changes need to be implemented they will be implemented," says McDonald.

Alvares is facing one count of criminal trespassing at a nuclear station. Additional charges are pending.

A statement from APS said in part:

At no time did the contractor have access to the protected area of the site, which is the inner security perimeter containing the reactor buildings and equipment important to the operation of the plant. Areas within nuclear power plants have multiple levels of security based on their significance. These security measures include armed security officers, physical barriers and detection zones, and a rigorous series of background investigations for individuals. Each increasingly more sensitive area of a nuclear plant requires higher and higher security clearance.

The primary purpose of the vehicle checkpoint is to search for weapons, explosives or other prohibited materials rather than to screen individuals. The driver of a vehicle seeking to enter the property through the vehicle checkpoint must present a valid government-issued ID and that individual can confirm identity for other individuals in the vehicle.

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