LINDEN, Calif. -- The serial killer's crude map pointed to a spot near a stand of oak trees, off a lonely stretch of road through rolling green fields here in California's Central Valley. The "boneyard" was there, at the bottom of an abandoned well, he wrote.
Last week, investigators digging at the well site uncovered more than a thousand bones, including human skull fragments, as well as a woman's purse, shoes and jewelry. But even as state crime lab technicians examine these latest remains, the investigators have learned of two more burial sites that may hold a dozen or more victims.
The clandestine burials were the work of Wesley Shermantine and Loren Herzog, dubbed the "speed freak killers" after a 15-year drug-fueled killing spree that ended with their arrest and conviction for multiple homicides in 1999. The two were childhood friends who grew up and lived in Linden, a small farming community.
Authorities charged them with six murders but suspected them of killing 20 or more. Only a few bodies of victims were found at the time.
In late January, however, Shermantine, on death row at California's San Quentin prison, sent the first of several letters containing maps and directions to burial sites to an area newspaper, after Leonard Padilla, a Sacramento bounty hunter, offered him $33,000 for the information. Following the directions, authorities started excavating the second week of February. By Feb. 12, they had recovered the remains of at least three people.
But some close to the case believe that Shermantine has yet to reveal the location of the two killers' main burial sites.
"It doesn't make sense for him to disclose the mother lode at first," said Thomas Testa, a San Joaquin County deputy district attorney who prosecuted Shermantine and Herzog in the late 1990s. "It seems more in line with his stated position to give morsels of information at first."
On Tuesday, Shermantine, who insists that he did not commit any murders but only assisted Loren Herzog in disposing bodies, confirmed the existence of additional sites in a new letter to the Stockton Record, an area newspaper. In the letter, one of many sent to the paper, he said he will provide directions to two final burial sites only after he begins to receive the money promised him.
Herzog, who was released from prison in 2010 after his murder convictions were overturned on a technicality, killed himself in January after Shermantine agreed to provide handwritten maps and directions to the burial sites.
"I'll give [the] last two areas and we'll recover all Herzog's victims," Shermantine wrote in a letter dated Feb. 13. "I will not send people on a fool's search."
According to Testa, these burial sites could yield the remains of a dozen or more additional victims. During the killers' trials, he introduced testimony by family members and associates that the two had bragged of "hunting the ultimate kill" -- humans -- and of committing 18 to 20 murders. Both men were linked to numerous violent assaults and attempted abductions during the 1980s and '90s.
"A lot of girls and young women went missing around this time," Testa said. "There were a lot of suspicious deaths that occurred during this period that we looked into but just couldn't make a case."
"Loren's boneyard will have many victims," he said.
Padilla, the bounty hunter who offered to pay Shermantine for directions to the burial sites, and his business partner, Robert Dick, are also convinced that authorities have not yet excavated the killers' main burial sites, they said.
Dick said the property owner of the land with the excavated well told him that it had been sealed in the early 1980s, shortly after he acquired the land.
"They didn't get up to speed with most of the victims until the late '80s and early '90s," Dick said. "So obviously it's another well. It's just common sense."
Investigators need to focus on another abandoned well marked on Shermantine's map, said Padilla, who is in regular contact with Shermantine by telephone.
Les Garcia, a spokesman for the San Joaquin County Sheriff's Department, which led the excavation, declined to comment on whether investigators were planning to excavate another well.
"Our investigators are continuing to work the case," Garcia said. "Our ultimate goal is that we locate as many human remains as possible so we can bring them home to their loved ones."
Yet Steve Moore, the San Joaquin sheriff, has faced criticism this month for slowing the search after he halted a plan by county prosecutors, California corrections officials and a retired FBI agent to temporarily release Shermantine from death row so he could guide authorities in person to the burial sites.
The sheriff's department was not involved in planning the operation and Garcia said it was halted due to safety concerns.
"The sheriff's reason for stepping in was the safety and security of the people in our county," he said.
But Padilla and Dick, who for years as bounty hunters have independently searched for the bodies and who played key roles in convincing Shermantine to disclose information about the burial sites, said Moore halted the operation because he was angry that he was not in charge of it. They said that they had subsequently been excluded from all aspects of the investigation.
"It's all about ego right now," Dick said. "It's not about the victims."
Testa, the prosecutor, said he sided with the bounty hunters. "They could have managed the security aspect of it well enough," he said. "Instead of looking for a way to make it happen, the sheriff seemed to look for a way to stop it."
According to Garcia, the sheriff's department remains open to the possibility of allowing Shermantine to guide investigators to the burial sites as a last resort.
Testa said bringing Shermantine out to the sites should be a top priority. "I think they should find a way to get him out and let him point these places out," he said. "If he's still willing to do it, I think they should move heaven and earth to get him out there."
The discovery of still more bodies could bring peace to many families, who have wondered for decades what became of their missing loved ones, Testa said.
One person who has learned the answer to some difficult questions is John Vanderheiden, the father of Cyndi Vanderheiden, who was 25 when she was abducted and killed by Shermantine and Herzog in 1998. Earlier this month, Shermantine's directions led investigators to a burial site on property once owned by his family. This week, the FBI confirmed through DNA testing that the remains were Cyndi Vanderheiden's.
"The search is over," John Vanderheiden said. "Once we get her home and get her buried and put in a safe spot, then I think we'll have closure."
Video produced by Andrew Rothschild
More:Loren Herzog Speed Freak Killers Victims Speed Freak Killers Wesley Shermantine Victims Speed Freak Killers Bodies
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