WASHINGTON (AP) - The federal government said Monday it cannot explain a reported incident of sudden, high-speed acceleration in a Toyota Prius on a San Diego freeway and acknowledged it may not be able to solve the mystery of what happened to the hybrid.

The National Highway Traffic Safety Administration said its engineers continue to investigate and are reviewing data from the Prius owned by James Sikes to try to understand what led to last week's high-speed incident. But so far, NHTSA says it has not been able to find anything to explain what Sikes reported.

"We would caution people that our work continues and that we may never know exactly what happened with this car," NHTSA said in a statement.

Inspectors tried during a two-hour test drive to duplicate the acceleration, but couldn't do so.

Sikes called 911 last Monday to report losing control of his Prius as the hybrid reached speeds of 94 mph. A highway patrol officer helped bring the vehicle to a safe stop. Though no one was injured, dramatic footage of the incident captured by local television stations captivated the nation, quickly becoming a high-profile headache for Toyota, which like NHTSA sent in an engineering team to investigate.

John Gomez, an attorney for Sikes, said the failure to repeat the incident is insignificant and not surprising.

"They have never been able to replicate an incident of sudden acceleration. Mr. Sikes never had a problem in the three years he owned this vehicle," he said Sunday.

But Rep. Darrell Issa, R-Calif., suggested the failure to duplicate the stuck accelerator, along with a vehicle design to prevent such occurrences, raises questions about Sikes' story.

"It doesn't mean it didn't happen, but let's understand, it doesn't mean it did happen," Issa said on CBS' "Early Show."

Toyota Motor Corp. planned to announce preliminary findings of its investigation at a news conference Monday in San Diego.

NHTSA is looking into claims from more than 60 Toyota owners that their vehicles continue to accelerate unexpectedly despite having their vehicles repaired.

Technicians with the NHTSA and Toyota could not duplicate what Sikes said he experienced March 8 on a mountainous but lightly traveled stretch of Interstate 8 east of San Diego, according to a congressional staffer's memo prepared for the House Committee on Oversight and Government Reform.

"Every time the technician placed the gas pedal to the floor and the brake pedal to the floor the engine shut off and the car immediately started to slow down," the memo read.

According to the memo, a Toyota official who was at the two-day inspection last week in suburban San Diego explained that an electric motor would "completely seize" if a system to shut off the gas when the brake is pressed fails, and there was no evidence to support that happened.

"In this case, knowing that we are able to push the car around the shop, it does not appear to be feasibly possible, both electronically and mechanically that his gas pedal was stuck to the floor and he was slamming on the brake at the same time," according to the memo.

Toyota has recalled millions of cars because of floor mats that can snag gas pedals or accelerators that can sometimes stick. Sikes' car was covered by the floor mat recall but not the one for sticky accelerators. He later told reporters that he tried to pull on the gas pedal during his harrowing ride, but it didn't "move at all."

The Prius is powered by two electric motor-generators and a small gasoline engine, all connected by transmission gears. A computer, which Toyota calls the "hybrid control computer" determines what combination of motors is needed and which would be most efficient.

Craig Hoff, a professor of mechanical engineering at Kettering University in Flint, Mich., said that for the Prius to accelerate out of control, at least two systems would have to fail simultaneously. They are the sensor signal that tracks the brake and gas pedal positions when the driver presses on them and the hybrid control computers.

"The chance of them both going wrong, plus the fact that the signal is bad, it just seems very, very, very remote," Hoff said. "Could it happen? Statistically, yes. But it just doesn't seem very likely."

Several events usually combine to cause problems with cars, and it's difficult to reproduce them, Hoff said.

"It's going to make it really hard to find, because you've got to line up the multiple effects," he said.

The congressional memo said both the front and rear brakes were worn and damaged by heat, consistent with Sikes saying that he stood on the brake pedal with both feet and was unable to stop the car. But if the fail-safe system worked properly, the brakes wouldn't have been damaged because power would have been cut to the wheels.

Gomez said the best evidence that his client was frantically slamming the brakes is that a California Highway Patrol officer who was giving Sikes instructions over a loudspeaker smelled burning brakes and saw the lights on.

Spagat reported from San Diego. AP Auto Writer Tom Krisher contributed to this report from Detroit.