WASHINGTON - After dissecting Toyota’s engine control software and bathing its microchips in every type of radiation engineers could think of, federal investigators found no evidence that the company’s cars are susceptible to sudden acceleration from electronic failures, the government said Tuesday.

The National Highway Traffic Safety Administration concluded that the sudden acceleration was caused by mechanical problems in some Toyota models — sticking accelerator pedals and floor mat interference — that it had previously identified as causes, reported The New York Times.

The findings, reached after a 10-month investigation, neither implicated Toyota nor exonerated it any further than had been the case after the earlier investigation.

Toyota eventually recalled more than 11 million Toyota and Lexus vehicles globally because of floor mats and sticky accelerator pedals. It also paid three fines totaling $48.8 million, because, the Transportation Department said, Toyota had not reacted appropriately to reports of problems.

“The jury is back,” said Ray LaHood, the transportation secretary. “The verdict is in. There is no electronic-based cause for unintended high-speed acceleration in Toyotas. Period.”

An engineer from the National Aeronautics and Space Administration, brought in to help conduct the inquiry, was slightly less categorical but still emphatic.

“It’s very difficult to prove a negative,” said Michael T. Kirsch, a principal engineer with NASA’s Engineering and Safety Center. But the electronic system for throttle controls in Toyotas would require two separate sensors to fail simultaneously in such a way that neither created an “error code” in the vehicle’s onboard computer.

There were relatively few instances of even one sensor failing, said Mr. Kirsch, who added that investigators had access to Toyota’s designs, engineering and warranty data.

Mr. LaHood and other officials were also quite diplomatic about a likely cause of the unintended accelerations — pushing on the accelerator instead of the brake. On Tuesday department officials called these “pedal misapplications,” and when a reporter asked if the problem was drivers making a mistake, Mr. LaHood shot back from the podium, “Nobody up here has ever insinuated the term that you used, driver error.”

In a statement, Steve St. Angelo, Toyota’s chief quality officer for North America, said the automaker hoped the study would help put to rest questions about the reliability of Toyota’s electronic systems.

Shares of Toyota rose 4 percent to close at $88.57, gaining momentum as news of the report leaked out ahead of the announcement on Tuesday afternoon.

The government said it was considering new research, on “the placement and design of accelerator and brake pedals, as well as driver usage of pedals, to determine whether design and placement can be improved to reduce pedal misapplication.”

It is also considering proposing rules, this year, that would require a standard method to turn off the engine so the driver does not have to insert a key into the ignition. It is also considering a requirement for “event data recorders,” a step it has long resisted. Many cars already have such recorders, simplified versions of an airplane’s “black box,” which capture data in the last few seconds before airbags are deployed, and keep information like engine speed, brake and accelerator application, and power of impact.

Wade Newton, a spokesman for the Alliance of Automotive Manufacturers, which represents most of the large carmakers, said his group did not oppose a requirement for the recorders. “Our biggest concern would be making sure there was proper lead time — particularly for automakers that have previously elected to not install E.D.R.’s in their vehicles,” he said. “They’ll need extra time for designing, engineering, fine-tuning and working with supply chain issues to put these devices into autos.”

The findings of the Toyota investigation were similar to those of a preliminary government report released in August. In fact, the finding vindicated not only Toyota but the safety agency itself.

“N.H.T.S.A., America’s traffic safety organization, was right all along,” Mr. LaHood said. He said the Transportation Department had ordered the search for an electronics problem because in the hearings on Capitol Hill, at which he testified last year, “just about every member of Congress didn’t believe that we had found the problem, which was floor mats and the sticky pedals.”

“As a former member of Congress, I thought we should listen to these members,” said Mr. LaHood, who represented a district in Illinois until President Obama named him transportation secretary. Speaking of his former colleagues, he said, “I hope they get the message today.”

In claiming victory, though, Mr. LaHood was far different in his tone toward Toyota than he was last year when news of the acceleration problems broke. At one point, during a Congressional hearing, Mr. LaHood said that owners of recalled Toyotas should stop driving the vehicles if they were having a problem and take them back to the dealers, though he quickly backtracked.

On Tuesday he praised Toyota for its plans to establish a $50 million safety center in Michigan.

In a statement, the highway agency said that NASA engineers had evaluated the electronic circuitry in Toyota vehicles and analyzed more than 280,000 lines of software code for any potential flaws that could initiate unintended acceleration.

As with the report on Tuesday, the preliminary examination given to Congress in August found no evidence of flawed electronics in vehicles that crashed. That examination found only one instance in which an accelerator pedal became trapped under a floor mat and none in which a pedal became stuck or sprang back too slowly.

The recalls have marred Toyota’s reputation for high quality and safe vehicles, hurting sales for much of the year. Toyota said its sales were down 0.4 percent in 2010; it was the only full-line automaker to report lower sales last year.

In response to complaints, Toyota has begun to install a brake override system, which allows the brake to stop the vehicle even if the accelerator is pressed simultaneously, as standard equipment across its lineup by the end of this year. The company also set up a panel so customer complaints are relayed more quickly to headquarters.