A fire at a derelict former indoor go-kart track at Tuckingmill was thought to be arson, according to fire investigators.
The blaze is the third in the area over recent days and it has prompted a warning about lives being put in danger.
Firefighters tackled the blaze on Tuesday at about 8pm.
A spokesman for Cornwall Fire and Rescue Service said that children were seen in the area of the fire on Tuesday and Devon and Cornwall Police have launched an investigation.
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