Sen. Chuck Grassley (R-Iowa) sent a letter Thursday to the National Highway Traffic Safety Administration, asking why the agency cleared Toyota given concerns that NASA raised when it investigated Toyota’s electronic systems.

Grassley cited issues with tin whiskers, first detailed by The Huffington Post in January, as one of the top concerns with NHTSA’s findings.

“Key questions about the cause of unintended acceleration remain unanswered,” Grassley wrote to David Strickland, the agency’s administrator.

Toyota recalled 8 million vehicles in 2009 and 2010 for sudden acceleration problems, a move that many considered a major black eye on the company’s reputation for making safe and reliable cars. Toyota blamed a few things for sudden acceleration: floor mats that came loose and pressed the gas pedal to the floor, and moisture in the gas pedal that made it stick open and gave the vehicle gas even when the driver wanted to stop.

Safety advocates argued that problems with Toyota’s electronics might also be to blame. So NHTSA brought in NASA to investigate the electronic systems.

NHTSA said the NASA report, released in February 2011, found nothing wrong with Toyota’s electronics. But a deeper reading of the report showed NASA scientists raised some concerns. Tin whiskers, which are tiny threads of tin that grow on tin wires and can cause electronics to behave erratically, are discussed on pages 17, 112 to 117, and 171 of NASA's 177-page report.

Between 2000 and 2010, NHTSA received nearly 10,000 reports from drivers that their cars -- not just Toyotas -- accelerated when the drivers wanted to stop, which is also known as unintended acceleration. Grassley said he wants to know how often NHTSA has looked into these cases and searched for evidence of tin whiskers.

“This is a serious issue,” he wrote.

Grassley also asked for more information on whether the highway safety agency has enough expertise to deal with these kinds of issues and how involved it was in directing NASA’s research.

"We have received the letter from Mr. Grassley and will review it carefully and respond appropriately," said Lynda Tran, a spokeswoman for the Department of Transportation, which oversees NHTSA. She also pointed out that NHTSA has a web page dedicated to Toyota's sudden acceleration issues.

In response to Grassley's letter, Toyota spokesman John Hanson focused on the lack of scientific evidence pointing to a problem with the electronic throttle control system, otherwise known as the gas pedal.

"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," he said.

Toyota's vehicles have fail-safe systems in place to deal with the "highly unlikely event" that tin whiskers will form and cause a problem, Hanson said. If something does occur, cars automatically go into what's called "limp home mode." That limits how fast the car can go, and drivers must consult a mechanic to figure out what's wrong.

This story has been updated to reflect comment from the Department of Transportation and Toyota.