Police said they responded to a call about 2:30 a.m. from an employee saying something horrible was happening at the facility in the city of Sagamihara, west of Tokyo.
A man turned himself in at a police station about two hours later, police in Sagamihara said. He left the knife in his car when he entered the station. He has been arrested on suspicion of attempted murder and trespassing.
Police said there were several casualties but did not provide any numbers.The Sagamihara City fire department says that 19 people were confirmed dead in the attack. The fire department said doctors at the scene confirmed the deaths.
A woman who lives across from the facility told Japanese broadcaster NHK that she saw police cars enter the facility around 3:30 a.m.
"I was told by a policeman to stay inside my house, as it could be dangerous," she said. "Then ambulances began arriving, and blood-covered people were taken away."
NHK reported the suspect, 26, is a former employee at the facility, Tsukui Yamayuri-en. Another broadcaster, NTV, said he broke into the facility by smashing a window with a hammer, and that he was upset because he had been fired, but that could not be independently confirmed.Television footage showed a number of ambulances parked outside the facility, with medical and other rescue workers running in and out.
Mass killings are relatively rare in Japan, which has extremely strict gun-control laws. In 2008, seven people were killed by a man who slammed a truck into a crowd of people in central Tokyo's Akihabara electronics district and then stabbed passers-by.
Fourteen were injured in 2010 by an unemployed man who stabbed and beat up passengers on two public buses outside a Japanese train station in Ibaraki Prefecture, about 25 miles (40 kilometres) northeast of Tokyo.
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