A global recall of nearly 2.9 million Toyota SUVs was prompted by a Canadian investigation into a crash that killed two people.
U.S. safety regulators revealed the probe in documents posted Thursday.
Last week Toyota announced the huge recall because the rear seat belts can be cut in a severe crash. If that happens, the belts won't restrain passengers.
Canadian investigators found the problem in crash tests after two rear-seat passengers died in a 2011 collision in Eastern Canada between a Toyota RAV4 and a Chevrolet Silverado pickup.
The front-seat occupants survived.
The unusual crash drew interest from Canadian regulators.
Tests done in October found that the belts could come in contact with a metal seat cushion frame.
Toyota says dealers will add plastic covers to the frame.
