Jack Earl visited his cousins just one week before the teenagers and their parents were killed in California in the fall of 1973. Earl, then 22, was devastated by the loss and lack of information about the murders. Earl, now a Roseburg resident, set out to learn more about the men who killed his relatives, a pursuit that soon became all consuming. “It totally became an obsession for me,” he said. “I feel so strongly about it, it’s like I’m the only person who can do it. A calling, so to speak.” Last year, Earl, now 61, published “Where Sadness …