What does "unsolved case" mean?

The Manchu-Mongolian unsolved case occurred in the 20th century. Japan signed an agreement with Zhang and his son to expand the railway authority and complete the operation of Manchuria.

After signing the agreement, the Japanese thought they had obtained Zhang's consent, so they implemented railways in China, and their ambitions were increasing day by day, which was beyond Zhang's control.

Later, in order to prevent injury, Zhang got rid of Japanese oppression with the help of American forces and raised funds to build a railway in China, which aroused Japanese dissatisfaction. They thought that Zhang's railway construction in the northeast seriously violated the original agreement, so they repeatedly threatened and warned Zhang, but Zhang turned a blind eye.

1928 At 5: 30 am on June 4th, the Japanese Kwantung Army planted explosives at Huanggutun Railway Station to blow up a special train, and Zhang was seriously injured. After being sent back to Shenyang, he died on the same day. This incident is called Huanggutun incident.

Extended data:

According to the existing case and the secret letter of Kwantung Army, Zhang was very dissatisfied because he failed to meet the various privileges of the Japanese in Northeast China. Because Yoshiichi Tanaka, then Japanese Prime Minister, refused to approve the Kwantung Army to disarm the Northeast Army, the Kwantung Army decided to plan an opportunity to murder Zhang privately, so as to disrupt the command system of the Northeast Army and throw the Northeast into chaos so as to send troops to occupy it.

Colonel Kawamoto, a senior staff officer of the Kwantung Army, made a detailed murder plan. Qin and Kenji Uehara, the director of the Fengtian Secret Service, participated in the planning, and the order was made by Lieutenant General Muraoka, then commander of the Kwantung Army. Later, because of the Huanggutun incident, Harutaro Murakami and Kawamoto were put into reserve.

According to "Memoirs of a Small Rocky", on June 1928, the masterpiece Kawamoto returned to Tokyo from the northeast of China, and privately revealed to Xiao Isoguo Zhao that he was the real murderer of Zhang. In his memoir published after the death of Kawamoto's masterpiece "I killed Zhang", he admitted to killing Zhang.

Baidu Encyclopedia-Huanggutun Incident