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

The staff of welfare facility Cosmos Earth likely forgot about the 19-year-old man while helping four other clients exit the vehicle at around 9 a.m. Thursday in Ageo, Saitama Prefecture, the police said.

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

Rescue workers said his body temperature at the time he was found was 41.4 degrees. The city of Saitama, which borders Ageo, reached 33.1 on Thursday, according to the local meteorological observatory.

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

"I apologize to him and to his family from the bottom of my heart," said Kenji Otsuka, the caretaker of Cosmos Earth.

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