A Mitsubishi Fuso Truck & Bus Corp. engineer compiled a report last year suggesting that the wheel hubs on the firm's tractor trucks broke due to structural defects and not improper maintenance as it had originally claimed, government officials said Friday.

The Mitsubishi Motors Corp. unit admitted on March 11 that wheels came off its large vehicles due to the defects, which caused a number of accidents, including one involving one of its tractors in January 2002 that killed a 29-year-old woman and injured her two sons.

In the report, the engineer said the firm failed to test the strength or durability of the wheel hubs during the design phase, and that the causes of the breaks had little to do with improper maintenance, the officials of the Land, Infrastructure and Transport Ministry said.

Mitsubishi Fuso changed its stance only after a series of accidents occurred involving broken wheel hubs of its tractors and other large vehicles. It offered to recall its vehicles for free repair.

Mitsubishi Fuso has never published the engineer's report, saying it is his personal research.

The officials said the ministry will have to determine at what time the firm knew about the structural defects.

The ministry has repeatedly asked Mitsubishi Fuso to submit explanations on the cause since the fatal accident in 2002. The firm had blamed the accident on improper maintenance until last week.

Kanagawa Prefectural Police sources said Thursday that MMC concealed information on defective front-wheel hubs and did not report the problem to the central government despite knowing about accidents caused by faulty products.