TUNKHANNOCK (AP) - A Pennsylvania supermarket employee who shot and killed three co-workers overnight posted a video online just before the shooting where he describes his plan in detail.

It's part of a long online trail prosecutors say 24-year-old Randy Stair left just before the killing at a Weis Market in Tunkhannock.In the video created May 11, Stair introduces two pistol-grip shotguns as "the twins" Mackenzie and Rachael and matter-of-factly describes how he needs to block the doors and who will be working the night of June 7.He also laments what a tight timeframe he has to block the doors, post all his documents and video on social media and then kill everyone.Police say Stair blocked store exits before killing three others and himself. A fourth co-worker escaped unharmed and called police.He ends the video pondering how to prevent something like this from happening again, and says to the camera: "you can't prevent it. You can only endure it."The partially animated film he produced praises teens who massacred students at Columbine High School in 1999.District Attorney Jeff Mitchell is calling 24-year-old Randy Stair's attack at a Weis Market in Tunkhannock, Pennsylvania, Thursday "a mental health situation that utterly spiraled out of control."The bio on a Twitter account the prosecutor said is believed to be Stair's says he "had to die in order to truly live" and that he was "speaking from before and beyond the grave."Police say Stair blocked store exits before killing three others and himself. A fourth co-worker escaped unharmed and called police.Randy Stair fired 59 shots from two pistol-grip shotguns he'd brought into the store in a duffel bag early Thursday.Police say one witness inside the store managed to flee and call 911.Police say Stair killed himself after shooting his three co-workers. It happened after the store closed for the night.Authorities say it's still unclear why Stair targeted 63-year-old Terry Sterling, 26-year-old Victoria Brong and 47-year-old Brian Hayes.