The death toll in Japan's worst bus accident in 25 years has risen to 15, police said Tuesday, as they struggle to determine why the vehicle veered off a mountain road it was not supposed to be on.

The passengers, mostly young skiers -- many in their teens or early 20s -- were asleep on the bus when it careened off the road before dawn Friday in the resort town of Karuizawa, northwest of Tokyo.

Fourteen people were killed and dozens injured in the immediate aftermath of the crash. The toll rose Monday with the death of a 21-year-old male university student, a local police spokesman told AFP.

Japan has suffered a series of high-profile bus accidents, some caused by drivers falling asleep at the wheel, though the state of the 65-year-old driver of the bus at the time of the latest crash remains unknown.

Both he and the bus's alternate driver were among the dead. Weather conditions are not seen as a factor.

Rules governing the working conditions for late- and long-distance highway bus drivers were tightened after an April 2012 accident that left seven people dead.

That vehicle hit a highway wall after its driver dozed off at the wheel. Many of its passengers were on their way to visit the Tokyo Disney Resort.

Japan has an extensive and well-maintained network of roads throughout its mountainous terrain, but the operating company acknowledged after the crash that the bus was supposed to be on an expressway as it headed to a ski resort.

Japanese media reports have speculated that intense price competition among tour operators and bus companies might have led to corner-cutting, such as trying to save expressway tolls by taking an unapproved route.

Japan's transport ministry will carry out emergency unannounced audits of bus companies across the nation this week, Transport Minister Keiichi Ishii told reporters Tuesday.