A man wielding a knife has reportedly killed 19 people and wounded 45 during a rampage at a facility for the disabled in the Japanese city of Sagamihara.
The attack took place on the grounds of the Tsukui Yamayuri-En Garden, with the attacker quoted by police as saying: "I want to get rid of the disabled from this world".
Police in Sagamihara, about 40km southwest of Tokyo in the Kanagawa Prefecture, have arrested 26-year old former employee Satoshi Uematsu, according to reports.
"The reason that so many people have died is because they were asleep," says RadioLIVE Japan correspondent Kelly Bolton.
"It was 2:30am and they were disabled, so some of them were probably wheelchair-bound or otherwise incapable of defending themselves, even if they were awake."
The 3-hectare facility was established by the local government and sits on the wooded bank of the Sagami River. It cares for people with a wide range of disabilities, Japanese media said, quoting an unidentified employee.
"This guy sounds completely insane. He was really angry and he decided he wanted to go and kill lots of people. So who does he attack? Disabled people while they're asleep," says Mr Bolton.
"He's a Japanese national, not an international terrorist - just a deeply disturbed and very angry person attacking the most vulnerable victims... The impression I got was that nobody knew this attack happened until he handed himself in because it happened so early in the morning."
Four of those injured are reported to be in a serious condition.
"There's little crime here and very little violent crime. Even street aggro, there's very little of it, very little arguing or confrontation in the streets. So this sort of incident is more unusual here than in other countries," says Mr Bolton.
"That said, Japan does have a large population - nearly 130 million people. There is a very small percentage of the population that do flip out occasionally, and this is one of those rare examples. The attacks usually involve knives, not guns, because people just don't have guns in Japan, much like in New Zealand."
Mass killings are rare in Japan. Eight children were stabbed to death at their school by a former janitor in 2001.