well they had plenty of suspects, and word was that they almost had enough to make an arrest a couple years ago, but ultimately the guy is still out there, or he died like you said. until they caught Gary Ridgway, a lot of people thought The Zodiac and the Green River killer were the same person.
that is according to the film. in real life, they never had enough evidence on one person to conclusively say he was the killer, although they had, and still do have, plenty of suspects.