A mentally disabled man died Thursday apparently due to heatstroke or heat exhaustion after being left in a service vehicle at a welfare facility in eastern Japan for about six hours in hot weather, police said.

Welfare facility staff likely forgot to help the 19-year-old man out of the van when assisting four other facility users in leaving the vehicle around 9 a.m. Thursday in Ageo, Saitama Prefecture, the police said.

The police said the man was found collapsed unconscious in the back of the van, which had three rows of seats and was parked near the welfare facility Cosmos Earth, around 3:25 p.m. He was rushed to hospital but died later.

When he was found, his body temperature was 41.4 C., rescue workers said. The temperature in Saitama which borders Ageo reached 33.1 C on Thursday, according to a local meteorological observatory.

The police are investigating the accident with a view to building a case of professional negligence resulting in death.

Kenji Otsuka, caretaker of the welfare facility, told reporters, "I apologize to him and to his family from the bottom of my heart."

According to the police, the facility noticed the absence of the man shortly before the facility was closed at 4 p.m.