TOKYO - A Japanese man who admitted murdering 19 people at a centre for the mentally disabled grinned at news cameras Wednesday before being questioned about the country's worst killing spree in decades.

Police searched the home of the 26-year-old, who reportedly said he wanted all disabled people to "disappear", after the knife rampage that left his victims in pools of blood, including some who were stabbed in the neck. With a blue jacket draped over his head, Satoshi Uematsu was escorted out of a police station into a waiting van before a crowd of flashing cameras. Inside the vehicle with the jacket removed, he smiled broadly in footage broadcast on morning news shows.

Uematsu's self-styled mission to rid the country of the mentally disabled - laid out earlier this year in a long letter that came to light Tuesday - has shocked Japan.

Questions were being asked about why he had been allowed to leave the hospital where he was admitted in February for mental evaluation following his explicit threats.

In a sign that the care centre feared its former employee, public broadcaster NHK - citing Kanagawa prefectural officials - said the facility in April set up 16 security cameras to watch out for him after he was discharged from the hospital.

An official at the Tsukui police station where Uematsu was held after the attack declined to comment on the investigation, only confirming that he was being transported to prosecutors for questioning.

Plainclothes police officers were seen searching his house where yellow tape declared it a no-entry zone. The two-storey dwelling is in the same neighbourhood as the care centre.

Local media said Uematsu has told police that he wants to apologise to bereaved families about the sudden loss of their loved ones, though he still justified what he did.

"I saved those with multiple disabilities," he told police, according to private broadcaster TV Asahi which cited investigative sources.

Uematsu broke into the care centre in the forested hills of Sagamihara in the early hours of Tuesday.

He reportedly tied up two caregivers before stabbing residents using a total of five knives - leaving a total of 26 people injured, 13 of them severely. He quickly turned himself in at a police station, carrying bloodied knives and admitting to officers: "I did it."

Uematsu reportedly also said: "The disabled should all disappear."

Security camera footage taken near the centre showed a vehicle arriving there shortly before the attack began. The driver opened the boot to remove objects before walking toward the facility.

At around 2:50 am, shortly after an emergency call was made to police from the centre, the footage shows the driver dashing back to the vehicle, carrying a large bag.

Uematsu left his job at the care home and was forcibly hospitalised in February after telling colleagues he intended to kill disabled people at the centre.

But he was discharged 12 days later when a doctor deemed he was not a threat.

He had previously delivered a letter to the speaker of the lower house of parliament in which he threatened to kill hundreds of disabled people, outlining a broad plan for night-time attacks against Tsukui Yamayuri-en and another facility.

In the rambling letter he presented a vision of a society in which the seriously handicapped could be euthanised with the approval of family members since "handicapped people only create unhappiness".

The top-selling Yomiuri Shimbun daily called the case "appalling" and urged a probe of the decision to release Uematsu from medical care.

"It is a matter of great regret for society to let such a serious stabbing incident happen," it said in an editorial which also called for increased security at care facilities.

Japan has one of the lowest rates of violent crime in the developed world. The killing spree is believed to be the nation's worst since 1938, when a man armed with an axe, sword and rifle went on a rampage that left 30 people dead.