The suspect, 26-year-old Satoshi Uematsu, was transferred earlier in the day from a local police station to the prosecutor's office in Yokohama.
The attacker left dead or injured nearly a third of the approximately 150 patients at the facility in a matter of 40 minutes early yesterday, Kanagawa prefectural authorities said. The fire department said 25 were wounded, 20 of them seriously.
Uematsu turned himself into police about two hours after the pre-dawn attack in Sagamaihara, a city about 50 kilometers west of central Tokyo.
Kanagawa prefecture welfare department official Shogo Nakayama said that officials from the Sagamihara facility confronted him about the letter a few days later, and Uematsu quit.
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His head and shoulders hidden with a blue jacket, the suspect was led out of a police station in Sagamihara on Wednes and into the back of an unmarked white van with emergency lights on top. Photographers and video journalists swarmed the van as it pulled away.
The parents of one of the seriously injured residents of the facility told Japanese television network NTV that their son is unconscious and on artificial respiration.
"I feel anger that he was a former worker," the mother said of the attacker. NTV did not identify the parents or show their faces.
Uematsu broke into the Tsukui Yamayuri-en facility by shattering a window at 2:10 a.M., according to a prefectural health official, and then set about slashing the residents' throats.
Further details of the attack, including whether the victims were asleep or otherwise helpless, remained unclear today.