James Sikes, 61, called 911 on Monday to report losing control of his Prius as the hybrid reached speeds of 94 mph. A highway patrol officer helped Sikes bring the vehicle to a safe stop on Interstate 8 near San Diego.
Federal and Toyota investigators who examined and test drove the car could not replicate the problems Sikes said he encountered, the memo said.
The findings raise questions about "the credibility of Mr. Sikes' reporting of events," said Kurt Bardella, a spokesman for California Rep. Darrell Issa, the top Republican on the House Oversight Committee.
The Wall Street Journal first reported the story Saturday.
The Journal said three people familiar with the probe, whom it did not name, said Sikes' brakes didn't show wear consistent with having been applied at full force at high speeds for a long period. Instead, they may have been applied intermittently, the newspaper said.
Toyota Corp. spokesman Mike Michels declined to confirm the report. He said the investigation was continuing and the company planned to release technical findings soon.
Michels said the hybrid braking system in the Prius would make the engine lose power if the brakes and accelerator were pressed at the same time.
Toyota has had to fend off intense public backlash over safety after recalls of some 8.5 million vehicles worldwide - more than 6 million in the United States - because of acceleration and floor mat problems in multiple models and braking issues in the Prius.
Regulators have linked 52 deaths to crashes allegedly caused by accelerator problems.
Transportation Department spokeswoman Jill Zuckman declined to comment on Saturday's report, saying investigators were still examining the data. The department oversees the National Highway Traffic Safety Administration, which is investigating the incident with Toyota.
Sikes called 911 from the freeway and reported that his gas pedal was stuck and he could not slow down. In two calls that spanned 23 minutes, a dispatcher repeatedly told him to throw the car into neutral and turn it off.
Sikes later said he had put down the phone to keep both hands on the wheel and was afraid the car would flip if he put it in neutral at such high speed.
The officer - who eventually pulled alongside the car and told Sikes over a loudspeaker to push the brake pedal to the floor and apply the emergency brake - said Sikes braking coincided with a steep incline on the freeway.
More on Toyota's Troubles:
Calif. Prius Driver's Story Stirs Skeptics
Calif. Prosecutor Sues Toyota Over Defects
Prius Driver in NY Crash Was Dealer-Bound
Prius Panic Drives Fear, Real and Imagined
Toyota to Expand Tundra Pickup Recall
911 Call Reveals Prius Driver's Terror
Toyota's Prius Recall Coming "Soon"
Toyota Fires Away at Acceleration Theory
House Panel Seeks Details on Toyota Recall
New Reports of Post-Recall Toyota Troubles
Once the car slowed to 50 mph, Sikes shut off the engine, the officer said.
Drivers of two other Toyota vehicles that crashed last week said those incidents also resulted from the vehicles accelerating suddenly.
The National Highway Traffic Safety Administration, which is probing Sikes' incident, is sending experts to a New York City suburb where the driver of a 2005 Prius said she crashed into a stone wall Monday after the car accelerated on its own.
And in Fort Wayne, Ind., the driver of a 2007 Lexus said it careered through a parking lot and crashed into a light pole Thursday after its accelerator suddenly dropped to the floor. That car was the subject of a floor mat recall. Driver Myrna Cook of Paulding, Ohio, said it had been repaired.