The "Dateline NBC" episode "Deadly Omission" investigates the murder of Casey Peek, revealing potential links to a decades-old unsolved mystery involving her husband, John Peek. As detectives delve into Casey's murder, they uncover connections to the suspicious deaths of John's previous wife, Carol Marlin, and her friend, Maggie Ginn. John Peek, portrayed as a manipulative figure, becomes the key suspect due to his complex relationships and control over those around him. The investigation highlights potential motives of financial gain and manipulation, ultimately leading to John's arrest and trial amid challenges of circumstantial evidence.