(CBS/AP) SAN FRANCISCO, Calif. - The abandoned well painstakingly excavated over the past six weeks in remote Northern California turned up no human remains, and the death-row inmate isn't talking anymore. So the FBI announced Thursday it is no longer actively looking for long-lost missing persons thought to be victims of the notorious "Speed Freak Killers."Led by the federal agency, a team of forensic experts and anthropologists on Jan. 7 descended on the well in a small town outside Stockton in hopes of recovering the remains of several victims of convicted killers Wesley Shermantine and Loren Herzog. The boyhood friends embarked on a 15-year, drug-fueled killing rampage through California's Central Valley and they claim dozens of victims before their arrests in 1999.
Shermantine is on death row awaiting execution after four murder convictions. Herzog committed suicide last year.
Most of their alleged victims have never been found. But with the promise of $30,000 from a Sacramento bounty hunter, Shermantine from his death-row cell last year directed investigators to the burial sites of five women and a fetus. Authorities also drove Shermantine around the area under heavy guard in August so he could point out additional burial sites.
The FBI chose an abandoned well in the town of Linden about 200 yards from another well that yielded the remains of three women and a fetus. Shermantine wrote letters to media outlets arguing investigators were excavating the wrong well.
On Thursday, the FBI dismissed his claims. The agency said two other sites that Shermantine said contained human remains also turned up nothing, and other directions from him also were misguided.
"Despite what Shermantine would like the media and the public believe, we continued to offer him every opportunity to assist with successful recovery of victim remains with the caveat that he must provide specific information regarding locations and identities," said Herb Brown, the FBI agent in charge of the Sacramento office.
"Since providing information that aided recovery of five victims in February 2012, his claims have lacked necessary specificity. He now refuses to meet with us, stymieing future investigation and excavation."
Brown said the FBI and San Joaquin Sheriff's Department spent a combined $200,000 excavating the 100-foot well, mostly by hand.
The sheriff's office excavated the first well, which yielded human remains, in February 2012. But it was criticized for the way it handled the first dig and called in the FBI to lead the excavation of the second well.
The sheriff's office used heavy machinery to dig up the first well, and the remains of the three bodies and the fetus found there ended up being mixed together, including those of JoAnn Hobson. Hobson's mother, Joan Shelley, filed a federal lawsuit in Sacramento this month accusing the San Joaquin Sheriff's Department of inflicting emotional distress for mixing up the remains.
Department spokesman Les Garcia said county lawyers are reviewing the lawsuit.
"Our position is the sheriff's office acted professionally and appropriately during the investigation and the recovery of victims remains," he said.
Garcia said the investigators remain hopeful more victims ultimately will be found. "We will continue to have future meetings with the FBI to discuss the ongoing investigation," he said.