The blaze yesterday afternoon was in a small outbuilding near the Fort Saumarez Martello Tower at L’eree.
The fire was started sometime shortly before 14:00 yesterday with witnesses saying a teenager had left the area shortly after smoke was seen billowing from the building.
Guernsey Police said: “At this time this incident is being treated as a suspected arson. The male has been described as white skin colour, around 14 to 15 years old, wearing a black hooded jumper and camouflage style trousers, he may have left on a black mountain bike.”
Any witnesses are asked to call Guernsey Police on 725111, speaking to PC 123 Cusack or Crimestoppers anonymously on 0800 555 111.
Pictured: The Martello tower near Lihou island.