Federal investigators concluded that a slow leak in a retreaded steer tire on charter bus led to an August 2008 Texas crash that killed 17 and injured 39 others.
While the National Transportation Safety Board did not fault the retreaded tire, it did recognize that placing retreads on steer axle positions violates federal laws.
The NTSB said the slow leak resulted in an underinflated condition, and the tire suffered a tread separation on I-75 near Sherman, Texas. The bus was carrying 55 passengers on a church-sponsored trip to Carthage, Mo.
Relatives of the victims are demanding Congress step in and expand oversight of the commercial bus industry.
NTSB investigators also found that the driver tested positive for cocaine and alcohol use, but did not cite that as a cause for the accident.