Safety investigators have found no evidence so far to support or disprove a California motorist's claim that his Toyota Motor Corp Prius sped out of control on its own, and cautioned that 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.

Toyota had no immediate comment on the NHTSA report, but planned to answer questions later at a news conference.

Scrutiny of the Prius raises the stakes for the Japanese automaker in ongoing U.S. regulatory and congressional investigations of unintended acceleration.

Toyota plans to slash its output of the gasoline/electric Prius by 10 percent starting this month due to a slowdown in sales from a peak last year, a source with knowledge of the plans told Reuters. Prius sales in February were down 40 percent from their October peak.

Jessica Caldwell, a senior analyst at Edmunds.com, said the pullback probably has more to do with keeping inventory lean in the narrow hybrid market rather than any meaningful fallout from Toyota recalls. The Prius still sells faster than other Toyota models, but U.S. inventories across the board backed up in February. Sales rebounded sharply in early March.

Caldwell said it appears Toyota is planning ahead for the impact of the all-electric Nissan LEAF and the Volt by General Motors Co, both due out later this year.

They're still a big fish in a small pond, Caldwell said. They don't want to put themselves in the position that the car is hard to sell. They don't want to lose that cache, she said.

Characteristically, the Prius has sold at a premium, but in March Toyota offered incentives, including a $750 rebate or zero percent financing for returning Toyota customers.

CALIFORNIA PRIUS PROBE

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

Police helped Sikes bring the car to a stop.

Two NHTSA investigators joined Toyota experts to examine Sikes' Prius. NHTSA said it drove the car and was reviewing readouts from the vehicle's data recorder.

The recorder, or black box, contains snapshots of information about speed, braking, throttle position and other parameters. Officials would not say whether that analysis has produced any useful information.

Investigators did say a system on Sikes' car that enables brakes to overcome acceleration when the gas and brake pedals are applied simultaneously worked properly in follow-up tests.

Police said the brakes were smoking when the officer who helped Sikes stop the car caught up to him on the freeway.

There was very little left of the car's brakes, NHTSA said.

Toyota has insisted that cases of unintended acceleration, when not caused by human error, were due to mechanical problems -- namely ill-fitting floor mats or a sticky accelerator pedal or both. Toyota has recalled more than 6 million vehicles in the U.S. -- 8.5 million worldwide -- for both since October.

Police said they have no reason to doubt Sikes, based on brake use and observations of the officer who helped him.

California Representative Darrell Issa, the top Republican on a congressional committee investigating the Toyota matter, said on NBC's Today show: It appears as though this is not an example that would likely be credible in finding a new flaw in the Toyota system.

He was cautious about suggestions that there could be some as yet undiscovered software problem acting as a ghost in the vehicle's acceleration system. Issa had a staff member on hand for the NHTSA examination of Sikes' car.

NHTSA has been criticized by congressional committees for not aggressively investigating past acceleration complaints against Toyota, including consumer reports that software-driven electronic throttles could be involved.

It has on numerous occasions examined Toyota and Lexus vehicles based on complaints similar to Sikes' and never found a problem with the acceleration system, outside of floor mats, records show.

Toyota steadfastly maintains its throttles are sound. Experts have said that unintended acceleration industrywide is a relatively rare occurrence and likely difficult to pinpoint.

Authorities believe floor mat problems in Toyota and Lexus vehicles are linked to five U.S. crash deaths since 2007, and they are investigating 47 other fatality reports over the past decade that allege unintended acceleration.

(Reporting by John Crawley; Editing by Gerald E. McCormick and Maureen Bavdek)

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