He was the mastermind behind the hijacking of Japan Air Lines Flight 351 to North Korea in 1970, although his arrest just days prior prevented his direct participation in the incident.
[3] As a result of this incident, the Japan Communist Party expelled Shiomi and all other known members of the Red Army Faction the following month.
"[5] Meanwhile, the group planned more substantive attacks against government agencies and the Prime Minister's official residence, for which they began "training" at the Daibosatsu mountain pass northwest of Tokyo.
[citation needed] In early 1970, Shiomi began making plans to hijack a Japanese airliner, codenamed "Operation Phoenix," that would allow group members to fly to Cuba and continue their training.
[7] However, the loss of Shiomi's ideological leadership, combined with relentless pursuit by police and continued arrests of key members, had taken a severe toll on the organization's cohesion.
Those that wanted to relocate overseas became the Japanese Red Army, led by Shigenobu Fusako and Tsuyoshi Okudaira, while those who wished to continue the revolution at home in Japan joined forces with the Kanagawa prefectural branch of the Revolutionary Left Faction to form the United Red Army, led by Tsuneo Mori and Hiroko Nagata.
Shiomi was incarcerated at Fuchū Prison and ended up spending a total of 19 years and 9 months behind bars (including pre-trial detention) before being released in December 1989.