BELLINGHAM, Wash. » Authorities say an SUV struck four Washington state teenagers while they were walking as part of a school physical education exercise, killing two of them and seriously injuring two others.

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

Francis says 18-year-old Shane Ormiston and 15-year-old Gabriel Anderson were killed. Two 17-year-olds — Michael Brewster and Kole Randall — were injured.

St. Joseph Medical Center in Bellingham says Brewster is being transferred to Harborview Medical Center in Seattle. The other boy is in serious condition.

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.