Investigators looking into Japan’s deadliest train accident in more than 40 years say they were treating the crash as the result of “possible criminal negligence,” according to news reports.

Authorities have searched several offices of West Japan Railway Co., seized documents and questioned a conductor who was supervising the train driver, who is among the casualties.

At least 106 people died in the crash, which may have been caused by the driver’s inexperience and the speed of the train. Another 450 people were injured and many are still not found.

Rescuers pulled three survivors out of the rubble a day after the crash, but have said they do not expect to find anyone else alive.

Survivors reported that the driver was speeding to make up lost time after he had overrun a previous station on the commuter rail line and then backed up the train.

The crash occurred at about 9:25 a.m. in Amagasaki, a suburb of Japan’s second largest city, Osaka.