Crime The Call That Changed Everything: Years After Mysterious Drowning, Woman's Brother Learns It Was No Accident For too long, Mary Horton Vail's younger brother believed she merely drowned while on a boating trip with her husband as he watched, unable to save her By People Staff Published on December 17, 2018 08:51 AM Share Tweet Pin Email For too long, Mary Horton Vail’s younger brother believed she merely drowned while on a boating trip with her husband as he watched, unable to save her. And then — after 30 years — he got a call that set him toward the truth. “Somebody from out of the blue called me one day and said, ‘You don’t know me from Adam, but [Mary Horton’s husband] Felix Vail married my daughter and before then he had a girlfriend,’ ” Will Horton recalls in Monday night’s episode of People Magazine Investigates, which focuses on Mary Horton’s death. (An exclusive clip is above.) The caller had more to share: In the years since Mary Horton’s mysterious killing, Felix’s other romantic relationships had come to tragic ends. “She’s disappeared,” the caller told Will of Felix’s girlfriend after Mary Horton. And then: “My daughter’s disappeared,” the caller continued, “and I think he murdered your sister.” “And I said, ‘Oh my God,’ ” Will says in the episode preview. How Mother of a Missing Teen Helped Bring America’s Oldest Suspected Serial Killer to Justice Courtesy Mary Rose; Courtesy Will Horton provided by USA Today Network; Courtesy Brian Hensley AP/REX/Shutterstock By the time a Louisiana jury found Felix Vail guilty of murdering Mary Horton, in 2016, he had left other missing women behind him: First Sharon Hensley, whom he called his wife, in 1973, and then Annette Craver Vail, whom he wed and who vanished in 1984. Felix was in his 70s when the jury reached its verdict in Mary Horton’s slaying and Louisiana prosecutors have said they believe he killed all three women, making him at the time the nation’s oldest suspected serial killer. (Neither Annette nor Hensley have been found and Felix has not been charged in their cases.) • Want to keep up with the latest crime coverage? Click here to get breaking crime news, ongoing trial coverage and details of intriguing unsolved cases in the True Crime Newsletter. “This is not a choirboy we’re talking about,” Calcasieu Parish District Attorney John DeRosier told PEOPLE in 2016 of Felix. “I’m convinced that there are other victims somewhere.” People Magazine Investigates: Gone Girls airs Monday (10 p.m. ET) on Investigation Discovery.