This was very preventable. Anybody who publicly threatens to commit mass murder should be kept locked up for a very long time, not weeks or months. They should only be released after mental health professionals are 100% confident that the person no longer poses a threat to society.
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UPDATE: Suspect threatened mass killings in letter
According to the city government of Sagamihara, where Uematsu’s house is located, he told other staffers of Tsukui Yamayurien on Feb. 18 this year, “I will undertake the mass murder of heavily disabled people at any time if I receive an order from the central government.”
On Feb. 19, the Tsukui Police Station, which had been notified about the threat from the facility, questioned Uematsu. At that time, he made the same threat.
Police reported the case to the Sagamihara city government on the basis of the Mental Health Law. The city government forced Uematsu to submit to mandatory hospitalization after undergoing a medical examination.
On Feb. 20, the hospital detected the presence of marijuana in Uematsu's blood and urine tests.
The hospital discharged Uematsu on March 2, saying that his symptoms had eased. At that time, he submitted a report to the hospital, which said, “I will live with my family after leaving the hospital."
UPDATE: Suspect threatened mass killings in letter