MELISSA HUNTLEY MOTZ: MURDER OR SUICIDE? JIM AVILA REPORTS ON THE MYSTERY SURROUNDING THE DEATH OF THIS YOUNG WOMAN. COULD HER HUSBAND BE TO BLAME?

PLUS: A CHANCE ENCOUNTER IN AN ONLINE CHAT ROOM LEADS TO ROMANCE, JEALOUSY AND MURDER. JIM AVILA REPORTS ON THE SURPRISING DETAILS THAT LED TO AN INTERNET LOVE TRIANGLE

BACK-TO-BACK EDITIONS OF "20/20" AIR FRIDAY, MARCH 19 ON ABC

On FRIDAY, MARCH 19, ABC News' "20/20" will tell the stories of two real-life murder mysteries in two back-to-back editions of the program (9:00-10:00 & 10:00-11:00 p.m., ET).

9:00-10:00 p.m.: "20/20" investigates the death of Melissa Huntley Motz. Did this young woman commit suicide or was she murdered by her husband? The local police conducted a murder investigation that hit a dead end when prosecutors dropped the case. Filing an open records request, "20/20" re-opens the nine-year-old cold case and the evidence locker at the Rock Hill South Carolina Police Department, and finds the final report on the case with a surprising conclusion. "20/20" also returns to the scene of Motz's death and talks with key witnesses and law enforcement officials who were part of the initial investigation. Jim Avila reports.

10:00-11:00 p.m.: "20/20" reports on a real-life online murder mystery. A chance encounter in an online chat room leads to romance between a beautiful 18-year old girl and a handsome young marine. But what she doesn't know is that her "sweet sexy marine" is actually a 46-year old married man. And before it's over, passion will turn to jealousy, a love triangle� and murder.?? "20/20" investigates a mystery where nothing is what it seems. Jim Avila reports.?

Elizabeth Vargas and Chris Cuomo are the co-anchors of ABC News' "20/20." David Sloan is the executive producer.