Knife attacker was known to authorities

TOKYO—Satoshi Uematsu, the man who confessed to stabbing 19 mentally disabled patients to death in Japan on Tuesday, was a former employee of the residence where the murders took place. He also was known for months to law enforcement and mental health authorities. They foresaw trouble but proved powerless to stop it, and now many Japanese are asking why.

Mr. Uematsu appears to have slipped through cracks in the systems that, in Japan as in other countries, are supposed to identify and treat mentally ill people and keep those with the potential for violence from harming others.

Although the authorities appear to have responded promptly to instances of ominous behavior by Mr. Uematsu, legal specialists, advocates for disabled people and members of the news media are questioning whether those authorities did enough to monitor and treat an apparently troubled man who had advertised his willingness to kill.

He had worked at the residence until February, when he delivered a letter to Parliament outlining a bloody plan to attack two facilities for the handicapped and saying all disabled people should be put to death.

He wrote that he intended to kill disabled people and that his plot would benefit Japanese society. The facility where he worked was so unnerved, it confronted him. He quit the job and police sent him to a psychiatric hospital, but doctors deemed him safe to release 12 days later. In the months that followed, his former workplace increased security, adding cameras to watch the buildings where 150 mentally disabled people resided. But he was left alone, free, unmonitored.