The media offered conflicting explanations involving suicide and murder, while the police did not publicly report the results of their investigation, which was then ended.

Under the Dodge Line policy of the Japanese government, Shimoyama was responsible for drastic personnel cutbacks of JNR, as a part of which on 4 July 1949 he released a list of about 30,000 employees to be fired.

[1] The cutbacks were ordered abruptly by the US government and enforced by the US military officials based in Japan as a way to reduce the political power of the then-Japanese Communist Party.

[citation needed] On the morning of the next day, 5 July, Shimoyama left his home in Ōta, Tokyo around 8:20 in his 1941 Buick.

His failure to arrive, after confirming from home that he would be there, caused a great stir at JNR headquarters, and the police were contacted.

[1] After the disappearance, an individual fitting Shimoyama's description was reported seen first in the Mitsukoshi department store in Nihonbashi,[3] then riding on a Ginza Line subway train bound for Asakusa Station.

On the other hand, a Tokyo municipal coroner who had examined the body at the scene believed that Shimoyama's death was suicide.

Keio University professor Nakadate Kyūhei believed that Shimoyama was alive when hit by the train, although he never saw the body.