SAGAMIHARA, Japan — A young Japanese man went on a stabbing rampage Tuesday at a facility for the mentally disabled where he had been fired, officials said, killing 19 people months after he gave a letter to Parliament outlining the bloody plan and saying all disabled people should be put to death.

When he was done, Kanagawa prefectural authorities said, 26-year-old Satoshi Uematsu had left dead or injured nearly a third of the almost 150 patients at the facility in a matter of 40 minutes in the early Tuesday attack. It is Japan’s deadliest mass killing in decades. The fire department said 25 were wounded, 20 of them seriously.

Security camera footage played on TV news programs showed a man driving up in a black car and carrying several knives to the Tsukui Yamayuri-en facility in Sagamihara, 30 miles west of Tokyo. The man broke in by shattering a window at 2:10 a.m., according to a prefectural health official, and then set about slashing the patients’ throats.

Sagamihara fire department official Kunio Takano said the attacker killed 10 women and nine men. The youngest was 19, the oldest 70.

Details of the attack, including whether the victims were asleep or otherwise helpless, were not immediately known. Kanagawa prefecture welfare division official Tatsuhisa Hirosue said many details weren’t clear because those who might know were still being questioned by police.

The suspect calmly turned himself in about two hours after the attack, police said.

Uematsu had worked at Tsukui Yamayuri-en, which means mountain lily garden, from 2012 until February, when he was let go. He knew the staffing would be down to just a handful in the wee hours of the morning, Japanese media reports said.

The facility employs more than 200 people, including part-timers, with nine of them working the night of the attack, Hirosue said. All those killed were patients.

‘‘They were working at night and got questioned by police after witnessing graphic violence, making them a little emotionally unstable now,’’ he said.

Not much is known yet about his background, but Uematsu once dreamed of becoming a teacher. In two group photos posted on his Facebook, he looks happy, smiling widely with other young men.

‘‘It was so much fun today. Thank you, all. Now I am 23, but please be friends forever,’’ a 2013 post says.

But somewhere along the way, things went terribly awry.

In February, Uematsu tried to hand-deliver a letter to Parliament’s lower house speaker that revealed his dark turmoil. It demanded that all disabled people be put to death through ‘‘a world that allows for mercy killing,’’ Kyodo news agency and TBS TV reported. The Parliament office also confirmed the letter.

Uematsu boasted in the letter that he had the ability to kill 470 disabled people in what he called was ‘‘a revolution,’’ and outlined an attack on two facilities, after which he said he will turn himself in. He also asked he be judged innocent on grounds of insanity, be given $5 million in aid and plastic surgery so he could lead a normal life afterward.

‘‘My reasoning is that I may be able to revitalize the world economy and I thought it may be possible to prevent World War III,’’ the letter says.

The letter was delivered before Uematsu’s last day of work at the facility, but it was unclear whether the letter played a role in his firing, or even if his superiors had known about it.