DETROIT — The government's top auto safety official is denying a U.S. senator's claim that his agency failed to thoroughly investigate unintended acceleration problems in Toyota cars and trucks.

Republican Sen. Charles Grassley sent a letter to the National Highway Traffic Safety Administration earlier this month saying that key questions remain about whether electronic glitches caused Toyota vehicles to accelerate on their own. He questioned whether NHTSA has enough expertise in electronics and why the agency called in NASA scientists help investigate.

NHTSA chief David Strickland, in a letter released on Friday, told Grassley that his agency has enough expertise to handle investigations, and that NASA was called in to take a second look. Scientists from the space agency also couldn't find any electronic causes of Toyota's runaway acceleration problems. The government concluded in February of 2011 that driver error, sticky gas pedals and floor mats trapping accelerators caused the problems.

Starting in 2009, Toyota was plagued by numerous complaints that its cars accelerated on their own, causing crashes, injuries and even deaths. The company eventually recalled more than 14 million vehicles worldwide to fix problems with gas pedals and floor mats. The recalls tarnished the company's reputation for reliability and cut into sales. Only recently has Toyota shown signs of recovering.

Grassley, of Iowa, also questioned whether a phenomenon called “tin whiskers” inside the gas pedal assembly or other electronics could have caused Toyota's problems, citing information sent to his office by unidentified whistleblowers. The microscopic whiskers can sprout from solder on electronic devices, causing electrical shorts and glitches.

In most vehicles today, electronic throttle controls have replaced mechanical cables.

Grassley told NHTSA in his July 12 letter that information from the whistleblowers “raises concerns that the scope of the NHTSA and NASA investigations may have been too narrow.”

But Strickland wrote that NHTSA and NASA both explored “tin whiskers” in detail and found that it would take several electrical shorts to make the Toyota throttle controls open wide enough to cause unintended acceleration. The agencies found that multiple shorts did not occur while consumers used the vehicles, Strickland wrote.

The agency found a small number of cases in which “tin whiskers” might cause a small throttle opening, but not enough to cause unintended acceleration, he wrote. “This `jumpy' throttle condition ceases immediately as soon as the driver releases the accelerator pedal,” Strickland wrote.

“NHTSA is not aware of any UA (unintended acceleration) crashes in which tin whisker shorting has been identified as a cause or contributor,” he wrote.

Toyota has said that scientific evidence has confirmed that there are no problems with its electronic controls, and that there is no data to show that whiskers are more prevalent in Toyota vehicles than those of other automakers.

Toyota's acceleration problems waned for more than a year until late last month, when NHTSA asked it to recall 154,000 Lexus RX 350 and RX 450H SUVs from the 2010 model year. The agency said floor mats could cause unintended acceleration and told owners to remove the driver's side mats until the problems are repaired.

NHTSA also said it has asked Toyota for information about the SUV problems to see if it reported the problems quickly enough. In 2010 and 2011, Toyota paid a record $48.8 million in fines to the government for failing to promptly alert regulators to safety problems.

Toyota said it gave NHTSA information about the SUVs in a timely manner after investigators asked for it.

In other auto news, Mazda Motor Corp announced Friday it will recall 217,500 Tribute sport-utility vehicles sold in the U.S. to fix the same sticking gas pedal problem that Ford Motor Co. is dealing with in older Escape SUVs.

The vehicles were jointly developed and share the same design.

The Mazda recall affects Tributes from the 2001 to 2006 model years and the 2008 model. Like the Ford recall, only vehicles equipped with the 3-liter, V6 engine and cruise control.

Ford on Thursday recalled 421,000 Escapes sold in the United States. Ford said the problem occurs because of inadequate clearance between the engine cover and the cruise control cable. This can cause the throttle to stick when the accelerator pedal is fully or almost fully depressed. It plans to fix the problem by having dealers raise the engine cover by replacing a fastener. This will provide adequate clearance for the cable, Ford said.

The recall came just a week after the National Highway Traffic Safety Administration opened an inquiry into the problem, noting 68 complaints, including 13 accidents, nine injuries and one fatality. The issues affect Escapes from the 2001 through 2004 model years with the 3.0-liter, V-6 engine.

“Our internal investigation was already underway then, and we just completed our full technical investigation into the issue and the cause, which led us to taking this action,” Ford spokeswoman Marcey Zwiebel said.

The consumer group, the Center for Auto Safety, said that based on the age of the vehicles and the number of complaints, Ford has known about the problem for a long time. The group said that quicker action by the automaker might have saved the life of Saige Bloom, an Arizona teen killed in an Escape crash in January.

In a letter to NHTSA Administrator David Strickland, the group said that Ford's suggested fix for the problem was inadequate and asked regulators to impose the maximum fine of at least $17 million against the company for failing to recall the vehicles in 2005, when it was first aware of the problem.

The Los Angeles Times contributed to this report.