Safety investigators have found no evidence so far to support or disprove a California motorist's claim his Toyota Motor Corp Prius sped out of control on its own, and cautioned the case may never be explained, U.S. regulators said on Monday.

The National Highway Traffic Safety Administration (NHTSA) engineers drove the 2008 hybrid in an effort to recreate the episode of unintended acceleration reported by owner James Sikes, 61, but were unable to do so, the agency said in its first statement on the analysis conducted with Toyota.

So far, we have not been able to find anything to explain the incident that Mr. Sikes reported, NHTSA said. We would caution people that our work continues and that we may never know exactly what happened with this car.

Sikes told police and reporters he was driving the car on a freeway near San Diego last Monday when it surged forward unexpectedly on its own as he was passing another vehicle, and reached speeds exceeding 90 miles per hour.

Police helped Sikes bring the car under control and then to a stop. The incident, which officials said lasted 20 minutes and covered about 30 miles, added a new dimension to the already charged atmosphere surrounding Toyota's safety crisis over unintended acceleration.

(Reporting by John Crawley; Editing by Derek Caney)