journalist michael fleeman. >> harold gave several versions of what happened.tories begin to diverge is really what was exactly happening at the time that she went off the cliff. was she taking a picture of him? was he on his cell phone looking at text messages? >> reporter: friends and family weren't the only ones comparing notes and clues. because toni died in a national park, the fbi joined the investigation. and the more they looked at what happened, the more reasons they found for concern. starting with the trail itself. this is not an easy descent, and people who knew toni couldn't understand why a woman who had had bad knees since high school would even risk it. "dateline" retraced harold and toni's path with the help of joey thompson, a climbing and hiking guide who often works in rocky mountain national park. is this path a path well-traveled? do a lot of people come down here? it seems tricky. >> no. this path is way out of the way for any recreational hiker to be scouting about and having fun in the mountains. >> reporter: would you recommend somebod