A backup, emergency generator at the San Onofre nuclear plant might have been tampered with, plant operator Southern California Edison reported to the Nuclear Regulatory Commission this week.

The diesel generator in the Unit 3 reactor was found to have coolant in its oil system late in October, and the problem was reported to the NRC Oct. 30.

Both of San Onofre's reactor units remain offline, Edison says, so the problem presented no danger to plant workers or the public.

But on Thursday, Edison released a statement saying it had reported to the NRC on Nov. 27 that "an internal investigation found evidence of potential tampering as the cause of this abnormal condition."

"The evidence is not conclusive that it's tampering," said Edison spokeswoman Jennifer Manfre said Thursday. "But of course we are doing a very thorough investigation, so that has not been ruled out."

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