Investigation
The bank employees believed the thief was a policeman, and accepted his story about the bomb because threatening letters had been sent to the bank manager beforehand. The smoke and flames turned out to be the result of a warning flare he had ignited while under the car. At some point, the thief abandoned the bank's car and transferred the metal boxes to another car, stolen beforehand. That car too was abandoned, and the boxes transferred once again to another previously stolen vehicle.
There were 120 pieces of evidence left at the scene of the crime, including the "police" motorcycle, which had been painted white. However, the evidence was primarily common everyday items, scattered on purpose to confuse the police investigation.
A 19-year-old man, the son of a police officer, was suspected just after the robbery. He died of potassium cyanide poisoning on December 15, 1968. He had no alibi but knew nothing about police procedure. However, the money was not found at the time of his death. His death was deemed a suicide and he was considered not guilty, according to official record.
A massive police investigation was launched, posting 780,000 montage pictures throughout Japan. The list of suspects included 110,000 names, and 170,000 policemen participated in the investigation — the largest investigation in Japanese history.
On December 12, 1969, a 26-year-old man was suspected by the Mainichi Shimbun. He was arrested on an unrelated charge, but he had an alibi; the robbery occurred on the day he was taking a proctored examination. As the arrest was made based on false pretenses, the arresting police officer, Mitsuo Muto, was accused of abuse of power.
Read more about this topic: 300 Million Yen Robbery