PARIS (AP) — French police say two ski patrollers have been killed when the devices they use to trigger avalanches exploded as they were securing the slopes at a ski resort in the French Alps.

The high mountain gendarmerie of the Haute-Savoie region said the accident happened Sunday morning in Morillon before the ski slopes were opened to the public. Police officers are investigating the scene to determine the cause of the rare accident.

Ski patrollers are in charge of preventing potential dangers to skiers, notably by triggering controlled avalanches with powerful explosive charges.

The risk of an avalanche was estimated at "high" Sunday in Morillon, with a rating of 4 on a scale of 1 to 5 following a heavy snowfall overnight.