The head and operations manager of a charter bus company will be referred to prosecutors this week over a crash last year that killed 15 people, investigative sources said Monday.

The 55-year-old president of Tokyo-based ESP and its 48-year-old former manager are suspected of professional negligence and failing to take sufficient safety measures despite knowing that the driver, who was killed in the accident, had very little experience driving a large bus.

Hiroshi Tsuchiya, 65, struck a guardrail at 96 kph in the town of Karuizawa, Nagano Prefecture, on Jan. 15 last year.

The bus, which was en route to a ski resort in northern Nagano Prefecture with 41 people on board, veered off the road and rolled over, killing 15 and injuring 26.