Security has been beefed up at the San Onofre nuclear power plant in Southern California after it was discovered that a generator there might have been tampered with, says an report. The plant has been out of service since January due to a leak in a steam generator.

Southern California Edison said it discovered engine coolant in an oil system in the backup diesel generator in late October during routine monitoring, the report says. An internal probe found evidence of potential tampering, though it could not be confirmed.

The presence of coolant posed no safety risk, the report said.

The investigation into the potential tampering is ongoing, the utility said, adding that it has enhanced security at the nuclear power plant. The company informed the Nuclear Regulatory Commission of the issue on Oct. 30 and told the agency of the potential tampering on Nov. 27, according to the report.