Geneva ? Alpine ski resorts were on high alert Monday after heavy weekend snowfalls caused avalanches that killed at least five people in the last three days – mostly skiers who had ventured off marked runs.

In the Swiss resort of Lauterbrunnen, an American resident of Germany skiing well off the run was swept away by a snowslide and killed, police said. Police did not identify the victim but said a second American caught up in the avalanche escaped unharmed.

Another U.S. citizen died across the border in the French resort of Tignes after apparently falling into a lake while snowboarding on unmarked territory, rescue services said. The victim’s identity was not revealed, but authorities identified him as a 28-year-old Boston resident.

The force of the avalanche projected him through an 8-inch-thick layer of ice on Tignes Lake. The victim’s companion, a French woman, was injured.

In nearby Val d’Isere, a 19-year-old Swede who was also skiing off marked trails was swept away by an avalanche and killed.

Avalanches also were responsible Monday for a number of injuries in France and Switzerland, where four skiers were hurt near the popular resort of Verbier after being caught up in a 1,000-foot-wide snowslide.

“People by themselves can continue to easily activate avalanches,” the Davos-based Federal Institute for Snow and Avalanche Research said in its Monday evening bulletin.

On Saturday, a 45-year-old woman was killed while snow-shoeing near the Alpine resort of Les Diablerets. Together with Monday’s death near Lauterbrunnen, there have been 18 avalanche victims in Switzerland this season. Almost all of those killed by avalanches had ventured outside marked ski runs.

A 28-year-old Norwegian tourist also died in a landslide on an Italian mountain while sightseeing.

Switzerland was one of several European countries battered by blizzards over the weekend, when up to 2 feet of snow also closed roads, rail lines and airports in parts of Germany and France.

Most roads and rail lines were reopened by Monday. At least 13 people died in Germany from weekend traffic accidents and three more were killed in Switzerland in weather-related incidents.

A number of ski stations in France were shut over the weekend because of avalanche fears, but by Monday the risk had subsided in most areas. Three departments were still on elevated avalanche alert.