Safety experts have said vehicle electronic systems could be to blame for the problems that have led to Toyota's massive recalls but the review by the government, while still at an early stage, has not found any evidence of those problems.
Toyota, the world's largest automaker, has recalled about 9.5 million cars and trucks since October in a quality crisis that has threatened to undermine the Japanese automaker's reputation for building safe vehicles.
Following congressional hearings, the Transportation Department and NASA have been investigating what may have caused unintended acceleration in Toyotas. The government has received about 3,000 complaints about sudden acceleration and estimated the problem could be involved in the deaths of 93 people over the last decade.
The Transportation Department said it had not found any new causes of the problems beyond two previously identified in the recalls -- floor mat entrapment and sticking accelerator pedals.
Investigators with NASA and the National Highway Traffic Safety Administration have reviewed event data recorders, or vehicle black boxes, on 58 vehicles in which sudden acceleration was reported. In 35 of the 58 cases reviewed, the black boxes showed no brakes were applied.
In about half of those 35 cases, the accelerator pedal was depressed right before the crash, suggesting drivers of the speeding cars were stepping on the accelerator rather than hitting the brakes.
Fourteen cases showed partial braking. One case showed pedal entrapment and another showed that both the brake and the pedal were depressed. Other cases were inconclusive.
The black boxes are devices that track a number of details about a vehicle around the time of an accident, including which pedals were applied and how fast the car was traveling.
Olivia Alair, a Transportation Department spokeswoman, said the review of the black boxes was "one small part" of the investigation, which is expected to be completed later in the fall.
Alair said they were still at an "early period in the investigation" and experts with NASA and NHTSA were "conducting research at labs across the United States to determine whether there are potential electronic or software defects in Toyotas that can cause unintended acceleration."
Transportation Secretary Ray LaHood and NHTSA Administrator David Strickland briefed members of the House Energy and Commerce Committee on the findings of the government review.
Toyota has said its own investigation had found a number of explanations for the sudden acceleration, including pedal entrapment by floor mats, sticking gas pedals and the misapplication of the pedals. The company has said it had not found any cases in which the electronic throttle control was the cause.
Steve St. Angelo, Toyota's chief quality officer for North America, said in a live chat on the microblogging site Twitter earlier Tuesday that the automaker had identified two causes -- floor mat entrapment and sticking pedals. "We are confident that it's not the electronics," St. Angelo said in a tweet.
Toyota paid a record $16.4 million fine for its slow response to an accelerator pedal recall and is facing hundreds of state and federal lawsuits. The automaker has sought to address the problems by fixing millions of gas pedals in recalled vehicles.
Congress is considering upgrading auto safety laws in the aftermath of the Toyota recalls.