Investigators say the driver of an Iowa National Guard semi-trailer that rolled into a ditch in August and killed two soldiers was taking pain killers which could have significantly slowed his reaction-time. Johnston police says laboratory tests reveal that 20-year-old Specialist Dustin Colby of Mason City took three different forms of prescription depressants before the convoy he was driving in left Camp Dodge on August 27th. Johnston Police Sergeant Craig Porter says officers are not sure if Colby was tired, but when his semi-truck left the roadway, he did not react for more than a second, which is consistent with someone who has taken pain killers. Porter would not release the names of the drugs Colby consumed but added each carried a warning on the label about operating heavy equipment. Colby and his passenger, 30-year-old Staff Sgt. Bruce Pollema of Hull, were killed almost instantly in the accident.