A knife-wielding man stabbed and killed 19 people as they slept at a facility for the disabled in a town near Tokyo early on Tuesday, a senior government official said, Japan's worst mass killing in decades.
At least 25 other residents of the facility were wounded in the attack.
"This is a very heart-wrenching and shocking incident in which many innocent people became victims," Chief Cabinet Secretary Yoshihide Suga told a scheduled news conference in Tokyo.
Police have arrested Satoshi Uematsu, 26, a former employee at the facility in Sagamihara town in Kanagawa Prefecture, about 25 miles (40 km) southwest of Tokyo, a Kanagawa official said. At least one media report said Uematsu had called for euthanasia of the severely disabled.
Uematsu had turned himself in, the Kanagawa prefecture official, identified only by his surname of Sakuma, told an earlier news conference carried on public broadcaster NHK.
Another 25 people were wounded, 20 of them seriously, Sakuma said.
Kyodo news agency said the dead ranged in age from 19 to 70 and included nine males and 10 females. Earlier media reports had said as many as 45 people had been wounded.
Staff called police at 2.30 a.m. local time (1730 GMT Monday) with reports of a man armed with a knife on the grounds of the Tsukui Yamayuri-En facility, media reported. The man wore a black T-shirt and trousers, reports said.