PARIS — An avalanche in the French Alps killed four snowboarders, including an instructor, on Monday morning.

The four snowboarders — a 48-year-old man, his son, 15, and stepson, 19, and their 59-year-old instructor, all French citizens — were off-piste and preparing to descend a steep slope at the Tignes ski resort in the Vanoise range when the avalanche hit, officials said.

The accident occurred around 10 a.m., Nicolas Martrenchard, a representative of the national government in Albertville, a nearby city, said at a news conference.

Emmanuel Dubost, a National Police rescue worker in the nearby town of Courchevel, said in a phone interview that ski patrollers had first been alerted around 10:20 a.m. and reached the area of the accident shortly after. Other rescue workers, using a helicopter, landed at the site less than an hour later. Over 40 rescue workers, along with search dogs, were involved.