SAN FRANCISCO — Searchers following a map prepared by a serial killer on California's death row have unearthed human remains that could be one of his victims.

Wesley Shermantine was convicted of four murders and sentenced to death in 2001. Investigators believe he and childhood friend Loren Herzog, a pair known as the "Speed Freak Killers," may have been responsible for as many as 15 killings.

Herzog committed suicide last month after serving time for the killing of Cyndi Vanderheiden.

Shermantine recently offered to lead authorities to the burial spots of Vanderheiden and other victims on property once owned by his family.

Vanderheiden's father, John, says the San Joaquin Sheriff's Department called him Thursday to tell him they found a human skull. He believes it belongs to his daughter, who disappeared in 1998.