Two teenagers are dead and two others are in serious condition in Washington state after an SUV struck them while they were walking as part of a school physical education exercise.

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

An 18-year-old boy and a 15-year-old boy were killed.

Two other boys were taken to St. Joseph Medical Center. A hospital spokesman says the boys are in serious condition. They're both 17 years old.

Francis says the 34-year-old driver has been arrested for investigation of vehicular homicide and vehicular assault.

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