Corrected entry: If there have been multiple killings at Camp Crystal Lake and everyone, even the police, knew about it wouldn't they have closed it down for good by now?
Correction: The camp was closed down after the events of the first film. At one point in the second film, a character looks at a sign for Camp Crystal Lake and says "this place is on the same lake we're gonna be." In another correction someone mentions that Jason prowls for his victims all over the lake, which is a valid point. The only area that's closed is the camp itself, not the entire lake and town, which simply can't be closed down for good.
Also, if you check out the other side of the lake during the scene in the second movie where they go swimming right before Jeff and Sandra sneak away you can see buildings that could be other camps or lake homes owned by people like Chris' parents. Jason had these areas to find his victims, not just the camp from the first movie. Also, the officer that brings back Jeff and Sandra makes the statement that the camp was condemned.
Correction: The camp is closed down in Part 3 and is not reopened until Part 6 when they have renamed the area Lake Forest Green and Jason is in the ground buried. In this movie, the killings don't take place at the camp but at a house on Crystal Lake owned by Chris's parents. Throughout the series, Jason stalks the entire Crystal Lake area and not just the famous campgrounds. So, even if the camp is closed, Jason still has the rest of the area around the lake to look for victims. Can the police close that down too?
Tina Gilliam