>> i told them that david and i were swingers. >> and suddenly the investigation turned on its heel and headed in a whole new direction. >> they would meet couples on the internet. and her husband would prearrange it, then they would engage in some type of sexual relation with the couple itself. >> did that make your investigative mind click? >> the person who shot david shannon might be familiar with joan shannon. maybe the swinger lifestyle might have had something to do with that. >> finally, a possible lead. had the shannons' unusual sex life created a motive for murder? coming up, an intriguing new lead. >> david actually said, you need to meet my wife joan. was there a fatal love triangle in this circle of swingers? when "dateline" continues. [ giggling ] let's play dress-up.