SAGAMIHARA • Details that have emerged from investigations into Tuesday's massacre at a home for the disabled sketch a portrait of a deeply disturbed young man with violent ideas about ridding the world of disabled people.

Satoshi Uematsu, 26, was apparently still sending tweets around the time of the attacks that left at least 19 people dead at the Tsukui Yamayuri-En facility in Sagamihara town, about 40km south-west of Tokyo.

"May there be peace in the world. Beautiful Japan!!!!!!" said the last tweet sent from his account at about 2.50am on Tuesday, shortly after an employee of the facility had called the police, the Asahi Shimbun said.

Interviews with neighbours and posts on Uematsu's Twitter account paint a picture of an outwardly polite young man who became obsessed with the mentally and physically disabled people he helped care for in the home for more than three years until February.

In letters addressed to the Speaker of Parliament's Lower House in February, Uematsu said he could "obliterate" 470 disabled people.

"I could not stand idle as I thought about the exhausted look on the faces of their caretakers, the crazed look in the eyes of the staff working at the facilities, and, in the best interest of Japan and the world, I have been moved to take this action today."

The director of the home, Mr Katsuhiko Yoneyama, said he had a talk with Uematsu after learning about the letters, and the latter voluntarily agreed to quit.

The next day, the local authorities committed Uematsu to a psychiatric hospital, where he was diagnosed with marijuana-induced psychosis and a delusional disorder. But 12 days later, on March 2, the symptoms disappeared, and doctors concluded that he was not a danger to others, NHK reported.

Uematsu had studied to be a teacher. In 2011, he was a student teacher for third-graders at Chigira Elementary School, which he attended as a child.

Mr Akiyo Numasawa, vice-principal of the public school, said that Uematsu was "very gentle" and that there were no signs of mental illness or trouble.

Mr Akihiro Hasegawa, 73, who lived next door to him for eight years, said he had noticed nothing strange about Uematsu, a college graduate. "He always smiled when he greeted me. A really nice young man," he told Reuters.

"It would be easier to understand if there'd been a warning but there were no signs," said Mr Hasegawa, adding he thought Uematsu's experiences on the job might have affected his mind. "We didn't know the darkness of his heart."

NEW YORK TIMES, REUTERS