A missing American snowboarder was declared dead Sunday, raising the death toll to at least four in weekend avalanches that claimed the lives of daredevils seeking the dangerous thrill of skiing off-trail in virgin alpine snow.

Officials called off their search for the 31-year-old American a day after the snowboarder was swept away by a huge avalanche in the western province of Tyrol (search). It was unclear whether authorities had managed to recover the victim's body.

The snow slide that killed the snowboarder also killed two Canadians aged 40 and 57 at the popular resort of St. Anton.

The American was one of four U.S. citizens in their 30s caught in the avalanche; the three others managed to claw their way out of chest-deep snow. The Canadian was one of four people — two married couples — who were enjoying a ski holiday.

The avalanche (search), whose width was estimated as roughly equal to the length of three football fields laid end to end, struck at an elevation of about 7,550 feet in an off-trail area popular with thrill-seekers looking for deep powder.

A second, even larger avalanche later Saturday struck a party of five Germans snowboarding in the Gargellen region of the southwestern province of Vorarlberg, killing one and critically injuring two others. Officials on Sunday identified the dead snowboarder as a 25-year-old from Stuttgart who held joint U.S.-German nationality.

It quoted authorities as saying that group also was snowboarding off marked trails when the avalanche, estimated at 700 yards across, thundered down the mountainside. About 80 rescuers aided by dogs recovered the body and rescued the rest of the Germans.

Officials had raised the five-step avalanche alert to level four in recent days after a combination of heavy snowfalls, strong winds and subsequent mild temperatures made snow cover unstable and prone to breaking away.

Experts warn that skiing or snowboarding off-piste in unstable snow can be dangerous and triggers many of the hundreds of slides that annually claim scores of lives in Austria.

Officials who monitor avalanche conditions in the Alps issued a statement Sunday warning people not to venture off trails known to be safe, avoid skiing or snowboarding alone and approach all areas "with the greatest respect."