YELM, Wash. -

The van that exploded Tuesday in Yelm was modified with a homemade system designed to improve gas mileage.

A 55-gallon drum in the vehicle blew up, critically injuring a man and a woman who were taken to Harborview Medical Center in Seattle with burns.

Sgt. Ray Brady told KIRO the couple were unable to speak to detectives.

The system reportedly was designed for the van to run on gasoline fumes.