UPDATED 7.41pm: The man who murdered 19 people at a facility for the disabled in Japan sent a strange letter to parliament after he left his job at the centre in February this year.

LISTEN ABOVE: Washington Post's Tokyo correspondent Anna Fifield speaks to Chris Lynch

Police spoke to him at the time but nothing eventuated from it.

20 people are critically injured in a knife attack outside Tokyo, in a facility for the disabled.

The 26-year-old broke into the facility in the city of Sagamihara, near Tokyo, on Monday night and turned himself into police after his rampage.

Tokyo bureau chief for The Washington Post Anna Fifield told Rachel Smalley the attacker smashed a window with the hammer and then started on a stabbing rampage.

She said there were eight staff members on at the time.

"He apparently started stabbing people, was noticed by a staff member and someone came in and he somehow managed to fend them off, tie them up, grab their keys and then continue.

Ms Fifield said it can only be assumed that he knew the staff who were working there and many of his victims.

"So weirdly, immediately after the attack he drove himself to the police station where he turned himself and said 'I did it,' 'I wanted disabled people to disappear.'"

She said the knife was still in the car when he turned himself in.

"It's really shocking for Japan for an attack of this scale to happen, but even more so given that so many of the victims appear to have been elderly or bed-ridden, completely innocent and unable to defend themselves."

The suspect has been arrested for murder.

NHK says the suspect is a former employee at the facility.

CNN's Paula Hancocks said there's no word yet on a possible motive, but such violence is rare in Japan.

"What of course is shocking people about this one is the fact that the death toll is so high. There are suggestions that this could be one of the worst attacks, by a single perpetrator, in recent years."