SAN ONOFRE, Calif. -- Southern California Edison has changed its security procedures for inspecting train cars entering the San Onofre nuclear power plant after a sleeping illegal immigrant was accidentally carried onto the property, it was reported Thursday.

Edison officials said that a man was found on the San Onofre property -- but not inside its "protected area" where the nuclear reactors and fuel are located -- just after three freight rail cars arrived inside the plant around 12:30 a.m on July 25, the North County Times reported.

The rail cars carry freight inside the San Onofre grounds and pass by an area where Edison stores spent fuel -- highly radioactive material that can no longer produce power.

The accidental intruder was discovered when Edison employees spotted him standing near one of the recently arrived freight cars, John Todd, director of security at San Onofre, told the Times.

"He claimed he was hitching a ride from Oceanside to San Clemente and was sleeping in the car," Todd said, adding the man seemed surprised to find himself at the nuclear plant.

Brian Katz, Edison's vice president of nuclear oversight and regulatory affairs, said that the freight cars stand about 12 feet above the ground and that Edison has used mirrors on poles to inspect the cars before they are allowed onto the property, the Times reported.

"We went around that car twice with the mirror," he said. "We have concluded that we cannot do mirror inspections anymore." Citing security concerns, he declined to say how the procedures have changed.

Plant officials said that the train rider, whom the FBI deemed not to be a threat to the site, was detained and turned over to the Border Patrol, the Times reported.