The Toyota sudden acceleration debacle may be a thing of the past, but one U.S. Senator thinks the government investigation is worth another look. Sen. Charles E. Grassley claims the National Highway Traffic Safety Administration, along with NASA, may have let what he calls a "serious issue" fall through the cracks.

In a letter sent to NHSTA administrator David Strickland, Grassley raised questions about the investigation, asking whether vehicles were tested for a very serious defect called "tin whiskers," reports the Los Angeles Times. Tin whiskers, as described in a NASA report, are "electronically conductive, crystalline structures of tin that sometimes grow from surfaces where tin is used as a final finish." According to Grassley, who reportedly received tips from informants, a NASA report found evidence of the whiskers in one Toyota pedal assembly, causing a short circuit. A short like that, Grassley wrote, could cause the electronic system to become vulnerable to another fault that could "theoretically lead" to unintended acceleration.

NASA, however, was called into the initial investigation to determine whether electronic defects or software code errors were to blame, and after the 10-month probe, the NHTSA and NASA found no evidence. Mechanical problems, such as the floor mats causing sticky gas pedals, were concluded as the only defects to cause such an event.

"There have been a lot of theories that the tin whiskers can cause unintended acceleration. The fact is, they never have in a Toyota vehicle. The handful of cases where tin whiskers have been identified with the potential to cause a resistive short have led to deceleration when the fail safes caused the vehicle to go into limp-home mode," a Toyota spokesman told us.

The Japanese automaker recalled millions of vehicles for defective floor mats and sticky gas pedals following a fatal crash involving a 2009 Lexus ES 350, which investigators believe could have been caused by an improperly-installed floor mat that trapped the gas pedal.

What do you think of Senator Grassley's theory? Are tin whiskers at the root of Toyota's recall woes, or are the findings by NHTSA and NASA proof positive that electronics aren't to blame?