Complex sentence: Initially, some Japanese manufacturers suspected that the seat belt failures were a result of user abuse, rather than a design failure; however, the nine-month investigation by NHTSA concluded that the cause of the defect was that the buckles were made of ABS plastic.
Split into simpler sentences: Initially, some Japanese manufacturers suspected that the seat belt failures were a result of user abuse, rather than a design failure. However, the nine-month investigation by NHTSA concluded that the cause of the defect was that the buckles were made of ABS plastic.