STOCKTON (AP) — Searchers following a map prepared by a serial killer on California’s death row unearthed human remains Thursday that could be one of his victims, the father of the victim said.

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.

John,Vanderheiden, the father of the victim, said the San Joaquin County Sheriff’s Department told him a human skull had been found. He believes it belongs to his daughter, who disappeared in 1998.

“We hope it’s her so we can finally bring her home,” Vanderheiden said. “If it’s not, then it’s another of their victims.”

San Joaquin Sheriff’s Department spokesman Les Garcia said the skull was turned over to the California Department of Justice for identification, and searchers would continue scouring the property for the next several days. Two dogs trained to sniff out human remains made the discovery in a remote area of Calaveras County, Garcia said.

Sacramento bounty hunter Leonard Padilla said Shermantine sketched the map in his Death Row cell after the two spoke on the phone last week. Shermantine wanted assurances that the bounty hunter still intended to pay him $18,000 if he disclosed the location of Vanderheiden’s resting place, Padilla said.

Padilla made the offer three weeks ago. But a planned search that would have Shermatine lead the California Department of Corrections and Rehabilitation to the location was called off after San Joaquin County Sheriff Steve Moore objected to being left out of the loop. Moore said he was concerned with safety issues.

Authorities feared Shermantine would stop cooperating because of the delay. But after Padilla guaranteed him last week that his offer was still good, he said Shermantine drew a detailed map that ended up leading searchers to the human remains.

“The map is spot on,” Padilla said.

Padilla has also offered Shermantine additional money for leading authorities to other burial sites in the area thought to hold other victims. Padilla said one other site is near the former Shermantine property, and the other is a well once use by cattle ranchers in San Joaquin County.

Garcia said the investigation was ongoing when asked about potential searches elsewhere.

Shermantine and Herzog are suspected of going on a two-decade killing spree during a methamphetamine-fueled spree that began shortly after they graduated from high school and lasted until their arrests in 1999.

Both were convicted of multiple first-degree murders. Shermantine, 45, was sentenced to death, and Herzog received a 78-year sentence, which was reduced to 14 years after an appeals court tossed out his confession as illegally coerced.

Herzog was released on parole in 2010. He committed suicide in Lassen County after the Sacramento bounty hunter called and told him that Shermantine was disclosing locations of missing bodies and implicating him in the murders. Herzog was 46.