WHO KILLED JAY COOK & TANYA VON CUYLENBORG?

According to https://www.cbsnews.com/news/tanya-van-cuylenborg-jay-cook-cold-case-timeline/, the couple formed by Jay Cook and Tanya Von Cuylenborg were killed between November 18 and 19, 1987 near Bellingham, in Washington state. The two had moved from Port Victoria in Canada with the Ford Club van with the intention of going to Seattle to buy some parts (an oven, apparently) from