(CBS/AP) SAN FRANCISCO, Calif. - The FBI on Monday began the excavation of an abandoned well in central California in a renewed search for possible victims of two men known as the "Speed Freak Killers."
The FBI is leading the new excavation effort after the San Joaquin County Sheriff's Department requested help.
The sheriff's department was criticized for its handling of a previous excavation of another abandoned well that yielded the remains of three bodies and a fetus that authorities suspected were victims of Wesley Shermantine and Loren Herzog.
Two other bodies were found buried elsewhere around the same time. Four of the bodies have been identified as long-missing women suspected of being killed by Shermantine and Herzog. The fifth remains unidentified.
Local authorities say the pair went on a methamphetamine-fueled killing spree in the 1980s and `90s and might be responsible for as many as 19 deaths.
A team of the agency's forensic experts will be joined by local authorities, California State University, Chico anthropologists, and other investigators for the next few weeks to painstakingly dig up the San Joaquin County site mostly by hand, said Herbert Brown, the agent in charge of the FBI's Sacramento office.
Shermantine and his boyhood friend Herzog were arrested in 1999 and convicted of several murders each. Shermantine was sentenced to death after a jury convicted him of four murder charges.
Herzog's three murder convictions and 78 years-to-life prison sentence were overturned by an appeals court that ruled his confession was illegally coerced. He later pleaded guilty to voluntary manslaughter and was paroled in 2010.
Herzog hanged himself in January 2012 after Shermantine began directing authorities to grave sites.
Shermantine began cooperating with authorities after Sacramento bounty hunter Leonard Padilla promised to pay him about $30,000 for disclosing the location of bodies. Officials briefly removed him from his death row cell at San Quentin Prison to personally show investigators several sites in San Joaquin County.
San Joaquin County Sheriff Steve Moore asked the FBI for help in November after Joan Shelley said remains of her 16-year-old daughter, JoAnn Hobson, were returned to her mixed with bones from at least three other people who were discovered in Linden.
The FBI said Monday that heavy machinery would be used initially to clear soil covering the well and then the new dig will be done largely by hand.
"We all remain hopeful that our efforts at this site will ultimately return the remains of victims to their loved ones but know that such is not a certainty," Brown said.