Airing on "20/20," Friday, July 4 on ABC

Two bizarre freak accidents hit one family 17 years apart - were they just a horrible chain of events or was there a cold-blooded killer in the family? Plus, a man found dead in his hotel room. No sign of foul play, but ruled a homicide. Who is to blame? "I Know What You Did" airs on "20/20," FRIDAY, JULY 4 ( 10:00 - 11:00 pm ET) on the ABC TELEVISION NETWORK. (OAD: 10/25/13)

• Seneca County, NY… Two mysterious deaths, 17 years apart, each with a hefty insurance payout for one man: Karl Karlsen. Is Karlsen just really unlucky or the mastermind behind the death of his son Levi and his wife Christina? For the first time, Karlsen's own daughter speaks out about the nagging feeling she held for years that her own father was behind the 1991 arson fire that killed her mother and the 2008 freak accident that killed her brother, when a truck fell off a jack and crushed him. "20/20? is with investigators as they search for clues. David Muir with the inside story of Karlsen's murder trial.

• Greg Fleniken was found dead in his Texas hotel room, surrounded by cigarette butts and candy wrappers. At first glance, detectives thought the death of this well-liked 55 year old was from natural causes. But as they investigated, they discovered something more harrowing happened in that hotel room. "20/20? is with the private investigator hired by Fleniken's wife to help solve this puzzling case. David Muir reports.

David Sloan is senior executive producer of "20/20."