Japanese police have searched the home of the suspect in a mass stabbing spree at a facility for the mentally disabled in a neighbourhood west of Tokyo.
At his house in Sagamihara on Wednesday, police took in cardboard boxes to carry out any evidence. Parts of the property were sealed off with yellow police tape.
Suspect Satoshi Uematsu, 26, turned himself in to police early Tuesday, about two hours after the attack in the Tsukui Yamayuri-en facility that left at least 19 people dead and 25 others injured.
Kanagawa prefectural authorities said the attacker left dead or injured nearly a third of the approximately 150 patients at the facility in a matter of 40 minutes.
He had delivered a letter to parliament in February outlining his plan and saying all disabled people should be put to death.
It demanded that all disabled people be put to death through "a world that allows for mercy killing".
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Security camera footage played on TV news programmes showed a man driving up in a black car and carrying several knives to the facility.
The man broke in by shattering a window, according to a prefectural health official, and then set about slashing the patients' throats.
Details of the attack, including whether the victims were asleep or otherwise helpless, were not immediately known. However, all those killed were patients.
Al Jazeera's Harry Fawcett, reporting from Sagamihara, said the attack was "deeply disturbing" on so many levels: "The sheer scale and horrific nature of the attack, the twisted reasoning that apparently lay behind it and the fact that he set out in such specific detail what he intended to do and was still able to do it."
Uematsu had worked at Tsukui Yamayuri-en, which means mountain lily garden, from 2012 until February, when he was let go.
Uematsu asked he be judged innocent on grounds of insanity, and be given 500m yen ($5m in aid) and plastic surgery so he could lead a normal life afterwards.
"My reasoning is that I may be able to revitalise the world economy and I thought it may be possible to prevent World War III," the letter says.
The letter included Uematsu's name, address and telephone number, and reports of his threats were relayed to local police where Uematsu lived.
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Mass killings are rare in Japan. Because of the country's extremely strict gun-control laws, any attacker usually resorts to stabbings.
In 2008, seven people were killed by a man who drove a truck into a crowd of people in central Tokyo's Akihabara electronics district and then stabbed passers-by.
In 2001, a man killed eight children and injured 13 others in a knife attack at an elementary school in the city of Osaka.
This month, a man stabbed four people at a library in northeastern Japan, allegedly over their mishandling of his questions. No one was killed.
Source: Al Jazeera and agencies