Dozens of other residents were wounded in Tuesday's early-morning attack at the Tsukui Yamayuri-En facility for the disabled in Sagamihara.
He broke in by shattering a window at 2:10am, according to a prefectural health official, and then set about slashing the patients' throats.
The victims ages range from 19-70. They declined to reveal the contents of the letter, but Kyodo News quoted it as saying Mr. Uematsu wanted to carry out "euthanasia" on severely disabled people "to revitalize the global economy and prevent World War III", and describing a "mission plan" that resembled Tuesday's killings in some respects.
"This kind of incident is never heard of in Japan", Teruaki Sugimoto, a 66-year-old man who lives near the care home, told The Washington Post.
Uematsu, who gave himself up to police after Japan's worst mass killing in decades, said in letters he wrote in February that he could "obliterate 470 disabled people", Kyodo news agency reported. Later that night, Uematsu turned reported himself at the Sagamihara police station. He was carrying the bloodstained cloth and knife, officials said.
Uematsu had worked at Tsukui Yamayuri-en, which means mountain lily garden, from 2012 until February, when he was let go.
Uematsu, who was carrying a bag full of bloodied knives and blades of different sizes when he arrived at the police station after the frenzied attack, was living half a kilometer from the care facility, and is now undergoing mental health checks, sources close to the matter said Tuesday, as part of the polices investigation.
Police in Sagamihara City, Kanagawa Prefecture, about 40 kilometers southwest of Tokyo, have arrested Satoshi Uematsu, a 26-year-old former employee at the facility, Japanese media reported. "My goal is to euthanize, with their guardians' consent, seriously disabled people if they can't live at home or be active in society", he wrote, referring to the center by name. "I'm speechless. I don't know what to say".
The police officers arrived at the scene of the crime to find numerous people lying in passages, inside rooms and elsewhere at the facility, with stab wounds on their necks and elsewhere.
Comments to media by Yuji Kuroiwa, governor of Kanagawa Prefecture, during his visit to the facility on Tuesday also paint a telling picture.
Alleged killer Satoshi Uematsu was a former employee of the facility, and although reasons of his departure are still murky, speculations have arisen that he quit abruptly in February after being chastised about abusive remarks aimed at residents.
"He was just an ordinary young fellow", he said.
Japan incarcerates fewer of its citizens (55 per 100,000 people) than most other advanced countries, including Britain (149) and America (716).
Mass killings are rare in Japan. Because of the country's extremely strict gun-control laws, any attacker usually resorts to stabbings. Other methods for mass violence have been used, too: Arson killed 16 in Osaka in 2008, while a man drove a truck into a crowd in Tokyo that year before using a knife to attack people, ultimately leaving nine dead. " Exactly seven years before, on June 8, 2001, Mamoru Takuma killed eight children at the primary school where he'd previously been a janitor". The attack leads to increased security at schools.