'20/20' INVESTIGATES THE UNANSWERED QUESTIONS AND REMAINING MYSTERY SURROUNDING THE MASS SHOOTING IN LAS VEGAS

'20/20: 27 Days Later: Mystery in Las Vegas' Airs Friday, October 27 (10:00-11:00 p.m. EDT), on ABC

Following the mass shooting at Las Vegas' Route 91 Harvest country music festival leaving 58 people dead and some 500 others injured, "20/20" investigates the unanswered questions that still remain. The hour features ballistics and forensics experts attempting to unravel the perplexing mystery behind the deadliest shooting in modern American history. "20/20" reports on the shifting timeline of the attack; the search for gunman Stephen Paddock's motive; the tick-tock of his movements in the week leading up to the shooting; his missing hard drive; the hotel security guard's encounter with Paddock; and conspiracy theories and whether any of them have any validity. "20/20" also looks at the longtime girlfriend of Paddock, Marilou Danley. The hour includes interviews with Danley's brother, Paddock's friends and neighbors, and a litigator representing families of victims and survivors who have filed lawsuits. "20/20" follows up with a woman who lost her husband and survivors who are coping with their new normal. Correspondent Matt Gutman and chief investigative correspondent Brian Ross report. "20/20: 27 Days Later: Mystery in Las Vegas" airs on FRIDAY, OCT. 27 (10:00- 11:00 p.m. EDT), on ABC.

"20/20" is anchored by Elizabeth Vargas and David Muir. David Sloan is senior executive producer.