Authorities have started an on-site forensic investigation at a facility for the disabled, where at least 19 people were killed by a knife-wielding man who went on a rampage early on Tuesday.

The attack happened in Sagamihara, Kanagawa Prefecture, about 40 kilometers southwest of Tokyo, Japan. The 26-year-old attacker, Satoshi Uematsu, has already been arrested after turning himself in. Uematsu, who was a former employee at the facility, claimed he wanted to kill the disabled.

But the police say they are still investigating possible motives. The facility cares for people with a wide range of disabilities, and with a maximum capacity of 160 people, including the staff. The media reported that the facility is usually locked at night but the man broke into the building by smashing a window.