An SUV has struck four students in Washington state who were walking with their class as part of a physical education exercise, killing two teens and critically injuring two others.

State Trooper Mark Francis says the students were on a sidewalk Wednesday afternoon when the SUV left the road and hit them from behind.

An 18-year-old boy and a 15-year-old boy were killed in the crash north of Bellingham. Two other male teens were taken to a hospital with life-threatening injuries.

Francis says the 34-year-old driver told investigators he fell asleep at the wheel. Francis says the investigation will determine whether drugs, alcohol or other factors were involved.

The trooper says authorities believe the teens were walking on a raised sidewalk at the time of the crash.