"The fire was caused by children playing with fire," Inge Dahlø, from the local South Trøndelag police, told the VG newspaper.
Police investigating the fire on Thursday traced its origins back to an ice-skating event held at the local Sistranda school, later speaking to the mother of one of the children, who saw the children mistakenly set fire to the tinder-dry vegetation on the island.
"They were being careless with fire, despite clear instructions from the school management at the start of the race," Dahlø said. "Some of them found the temptation too great to resist."
The blaze, the second serious fire to hit Norway's north west coast in a week, was extinguished early on Thursday afternoon, after fire helicopters began to douse the flames.