Twin Peaks' biggest mystery revolves around the death of Laura Palmer and who was behind her brutal murder. The show begins immediately after Laura's corpse is found wrapped in plastic on a riverbank. Laura's death impacts the life of every single inhabitant of Twin Peaks, and the mysterious circumstances of her murder imply that even seemingly innocent characters were responsible for her tragic fate, at least in part, when the big picture is taken into account. Laura's death and the subsequent arrival of Agent Dale Copper are the first signs that Twin Peaks will never be the same again.

https://screenrant.com/twin-peaks-who-killed-laura-palmer/