The fire broke out in an industrial area in Gateshead, northeastern Britain, on Friday evening. A body was found in the building, believed to be a 14-year-old boy who was reported missing, the police say.
Eleven boys and three girls aged 11 to 14 have been arrested, suspected of manslaughter.
The police were alerted to the fire in the area at around 20:00 on Friday. The industrial area has declined in recent years with several abandoned buildings.
Criminal Commissioner Louise Jenkins says that what happened is an "extremely tragic event where a boy sadly lost his life".
The police are appealing to the public not to speculate about what happened, and said that the investigation into the fire is in its early stages.
