Engine coolant wound up in the oil system of a diesel generator at San Onofre unit 3, and Southern California Edison believes someone might have tampered with it.

Both units at the plant are offline following the discovery of premature tube wear in their steam generators, and the emergency diesel in question was itself down for regular maintenance. Nonetheless, the utility launched an investigation when the coolant was discovered. It reported the condition to the Nuclear Regulatory Commission inspector on site Oct. 30, according to an SCE release Thursday.

The evidence is not definitive, the utility emphasized, but it suggested potential tampering. As such, security has been ramped up at the plant and the investigation is ongoing. So far it has included testing, a review of plant logs and employee interviews.