( CNN ) -- U.S. Sen. Chuck Grassley , R-Iowa , is asking the National Highway Transportation Safety Administration to look into additional questions about unintended acceleration of Toyota cars . Citing unidentified whistle-blowers critical of `` too narrow '' federal investigations , the ranking member of the Senate Judiciary Committee asked the NHTSA in a letter to look into the phenomenon of `` tin whiskers '' -- or crystalline structures of tin -- that theoretically could lead to the unintended acceleration . The whistle-blowers also provided Grassley with documentation about the investigations by NHTSA and NASA into the Toyota vehicles , including one NASA report that stated : `` Because proof that the ( electronic throttle-control systems ) caused the reported ( unintended accelerations ) was not found does not mean it could not occur . '' Grassley 's letter to the safety agency 's administrator , David Strickland , asks the agency to provide the committee with all information relating to tin whisker testing and for the agency 's position on tin whiskers ' possibly causing the unintended acceleration . Strickland and his agency did n't immediately respond Thursday to a CNN request for a comment . A Toyota spokesman told CNN Thursday that `` so-called 'tin whiskers ' are not a new phenomenon and do not represent a mysterious or undetectable problem in a vehicle 's electronics . '' Last year , a federal investigation into possible causes of the unintended acceleration in Toyota cars found no fault with the automaker 's electronic throttle-control systems , the Department of Transportation said . So far there are three known causes of unintended acceleration in Toyota vehicles : improperly installed floor mats , sticky pedals and driver error . Toyota spokesman Brian Lyons said Thursday that `` all the scientific evidence '' confirms there 's no problem with the electronic throttle-control systems in Toyota vehicles . `` Indeed , no data indicates that tin whiskers are more prone to occur in Toyota vehicles than any other vehicle in the marketplace , '' Lyons said in an e-mail to CNN . `` Further , no one has ever found a single real-world example of tin whiskers causing an unintended acceleration event , nor have they put forth any evidence of unintended acceleration occurring in a Toyota vehicle because of tin whiskers forming inside an accelerator pedal position sensor , '' Lyons said . `` Toyota 's systems are designed to reduce the risk that tin whiskers will form in the first place and multiple robust fail-safe systems are in place to counter any effects on the operation of our vehicles in the highly unlikely event that they do form and connect to adjacent circuitry , '' Lyons said . In Grassley 's letter dated July 12 and released publicly Thursday , the senator asks Strickland why his agency relied on NASA engineers and whether the agency lacks `` the sufficient expertise to conduct such investigations and why . '' Said Grassley in the letter : `` This is a serious issue . From 2000 to 2010 , NASA concluded there were 9,698 identified UA ( unintended acceleration ) customer complaints and NHTSA concedes that this is likely only a fraction of the actual incidents of UA : 'NHTSA assumes that not all incidents are reported and that , accordingly , each complaint represents a greater number of unreported real-world failures . ' '' Grassley asks the agency how many of the 9,698 vehicles were inspected for tin whiskers . In March , CNN reported that Toyota engineers found an electronic software problem that caused `` sudden unintended acceleration '' in a test vehicle during pre-production trials , according to a company engineering document obtained by and translated for CNN . The 2006 document , marked `` confidential , '' recounted the results of an adaptive cruise-control software test in a model internally designated the 250L , a vehicle later sold as the Lexus 460 in Japan and Europe . The document says a `` fail-safe overhaul '' would be needed for another model in production , internally designated the 180L , which the company says was later sold as a Toyota Tundra . In response , Toyota asserted that the document shows no such thing , and the company continues to deny that any sudden unintended acceleration in any of its vehicles was caused by electronic systems . The Japanese-language document said that Toyota engineers recorded a `` sudden unintended acceleration '' in the 250L 's adaptive cruise control , which was designed to slow the vehicle if sensors detected an object ahead and accelerate the vehicle when the obstacle clears , according to a Tokyo-based translation house with expertise in automotive and technical matters that CNN retained to independently translate the document . Toyota insisted the translation was inaccurate . Despite multiple requests , the company did n't provide its own translation of the document . Neither the test vehicle nor the adaptive cruise-control system cited in the document has been sold in the United States , Toyota said . Grassley set a July 26 deadline for the NHTSA to respond to his eight sets of questions . CNN 's David Fitzpatrick contributed to this report .
Sen. John Stone , R - Iowa , receives documents from whistle - blowers . Grassley is the ranking member of the Senate Judiciary Committee . Questions bear upon ` ` tin whiskers '' possibly cause the acceleration . Toyota says ` ` tin ##kers '' do n't ` ` represent a mysterious ... force ''