>> investigators found five shell casings at the end of the exit ramp and another about 200 feet away. >> our forensic people were able to tell us that it most likely was an ak-47 type rifle, an assault rifle. >> knowing megan had been assaulted with this ak-47, again, it just sends chills of agony through my mind. >> investigators also noticed a fresh scrape along the driver's side of the car. it looked as if megan's car had been sideswiped. then investigators got a huge break. they learned that megan's rental car was one of the first to be equipped with an event data recorder. >> the event data recorder is a huge step forward in traffic safety. it's like the black box in an aircraft that gives us an opportunity to better understand how crashes happen.