On the morning after Christmas, Monty Hardy ran his Toyota Avalon through a stop sign, broke through a fence and clipped a tree before flipping the sedan into a pond in a Dallas suburb, fatally injuring all four occupants.

The Avalon was one of millions of Toyotas recalled in November because the gas pedal could get jammed by floor mats and accelerate uncontrollably.

But when the driver-side mat was found in the trunk, the accident fueled suspicions that there was more wrong with the Toyota than the risk of pedal entrapment.

Police in Southlake, Texas, closed the investigation last month without reaching any conclusion. They didn't rule out a defect that might have caused unintended acceleration, or the possibility that Hardy, 56, an epileptic, may have suffered a seizure.

"We couldn't exclude either of those as factors," said a spokesman, Sgt. Mike Bedrich.

The case illustrates how difficult it may be to prove or disprove allegations of unintended acceleration in Toyota cars and trucks. Every day, new lawsuits are filed in the United States against the Japanese giant, which may find itself mired in litigation for years.

As Congress prepares to open hearings this week on Toyota Motor Corp.'s handling of the recalls, legal experts say the company's liability may be determined in part by any revelations that emerge in the hearings and from an investigation by the U.S. Department of Transportation.