Satoshi Uematsu, a 26-year-old who had worked at the Tsukui Yamayuri-en facility in Sagamihara until February, broke in through a window early Tuesday morning before beginning his rampage, Kanagawa Prefecture officials said at a news conference.
The attack -- in which nine men and 10 women were killed, and 26 more people injured -- is Japan's deadliest mass killing since the end of World War II.
After tying up two members of staff at the care home, Uematsu went from wing to wing, floor by floor, killing patients, according to Lili Horiguchi, a Prefectural welfare division employee. He first killed five female patients on the first floor of the facility's east wing, before crossing to the male wings, killing nine more people.
The suspect turned himself in at a local police station around an hour after the attack began, carrying a bloodstained knife and cloth, officials said.
In a letter he wrote several months before the incident, seen by CNN, he said he had "the ability to kill 470 disabled people."
"I am aware that this is an outrageous thing to say," he wrote, adding that he dreamed "of a world where disabled people with severe difficulties socializing as well as severe difficulties at home are allowed to be peacefully euthanized."
Japanese national broadcaster NHK reported
that he had been committed to an institution earlier this year to prevent him from "harming others" following the submission of the letter. He had resigned from his job when he was committed but was discharged in March.
The hospital failed to inform the care center when he was released, despite a request to do so. Extra security cameras had been set up around the facility, and police were informed about a potential problem with Uematsu, then an ex-employee.
Uematsu, who worked at the facility from 2012 until earlier this year, previously worked for a transportation company, and had trained to be a teacher.
While working at the home, he was cited for his work attitude but had no specific problems with the patients, a Prefectural official said at a press conference Wednesday.
Former colleagues said he was personable and good with children. Neighbors were shocked to hear of his involvement in the incident.
NHK also reported that he had had run-ins with the police -- last year he had fought and injured a man at a suburban Tokyo train station.
The surviving patients at the facility slept there last night, in the wings that he did not enter and, in the case of the male residents, in the gymnasium.
The 7.5-acre site in the bucolic mountain town is home to 149 residents, ranging in age from teenagers to those in their 70s, according to the Kanagawa prefectural website. It can house a total of 160 people. Just under a third of those living at the facility are elderly.
A total of 222 people work at the facility, but only nine -- one of whom was a security guard -- were there when the incident occurred.
The incident sent shockwaves through Japan, where mass killings are rare.
In June 2001, eight children were killed
when a former janitor entered an Ikeda elementary school in Osaka and began stabbing students at random.
In June 2008, a man ran over a group of people with his truck and then stabbed 18, killing
seven, in Tokyo's famous Akihabara gaming district.
The last time Sagamihara made global headlines was in 2012 when Naoko Kikuchi, a member of the Japanese doomsday cult
responsible for the 1995 nerve gas attack on the Tokyo subway, was arrested there. The attack killed 13 people and injured more than 5,500 people.
The cult, Aum Shinrikyo, was responsible for another sarin gas attack
the previous year that killed seven people and sickened some 200 more.