Wednesday’s tanker truck explosion near a Santa Rosa Kaiser Permanente medical building is now under investigation by state and local officials. The truck was delivering liquefied oxygen to the medical center on Old Redwood Highway when the explosion occurred. Investigators from the California OSHA are joining the Santa Rosa Fire Department in determining what caused the blast that sent debris flying and resulted in evacuations and a brief closure of Highway 101. The unnamed truck driver was seriously injured and remains hospitalized in stable condition.