In a Dateline NBC exclusive, lead detectives speak out about the investigation into once-prominent South Carolina attorney Alex Murdaugh, convicted last March of murdering his wife Maggie and son Paul.

For the first time, South Carolina Law Enforcement Division investigators Britt Dove and David Owen speak out about the moment they discovered video evidence proving Alex was at the scene of the murders when they occurred. Dove explained to NBC News' Craig Melvin that after listening to the video "three to four times," he was in "disbelief." Owen added he knew at that moment the case had been blown wide open, saying: "I can prove that Alex was lying to me."

The Murdaugh Murders: Inside the Investigation also features an interview with the former Murdaugh family housekeeper Blanca Turrubiate Simpson, who opens up about her unique history with the Murdaughs, as well as Judge Clifton Newman, who presided over the murder trial. 

When 17-year-old Alissa Turney is reported missing in Phoenix, it takes years before detectives treat her case as a homicide. A new investigation unearths dark family secrets, leading to a dramatic courtroom drama and an outcome no one saw coming.

INTERVIEWS WITH: Detective William Andersen, Alissa's brother James Turney, Deputy County Attorney Vince Imbordino, and others.