By Ronnie Cohen

SAN FRANCISCO (Reuters) - Investigators searching for possible additional victims of California's "speed freak" serial killings of the 1980s and '90s have run out of places to look, after the latest excavation of a suspected burial site came up empty, the FBI said.

No human remains were found in the abandoned, 100-foot-deep well dug out by hand during a six-week operation on a patch of pasture land near the San Joaquin County town of Linden, east of San Francisco, the FBI said on Thursday.

The well shaft, mostly filled with rock-hard soil, was about 200 yards (meters) from a spot identified last summer by condemned killer Wesley Shermantine as a probable burial site for some of those killed by him and his now-deceased accomplice, Loren Herzog.

"We are disappointed that our efforts did not yield remains, offering closure to families who suspect that Wesley Shermantine and Loren Herzog were involved in the disappearance of their loved ones," FBI Special Agent Herb Brown said.

The FBI said Shermantine was asked through his attorney last week for additional leads but refused to talk. Brown said he would renew search efforts if Shermantine offered specific new information about locations and identities of remains.

For now, however, Shermantine "refuses to meet with us, stymieing future investigation and excavation," Brown said.

Shermantine and Herzog have long been suspected in as many as 22 deaths, mostly of young women and girls who vanished decades ago.

Ultimately convicted in connection with six killings, they were dubbed the speed freak killers for the methamphetamine-fueled violence they unleashed in and around California's farm-rich San Joaquin Valley.

Shermantine suggested in letters to reporters and others last year that the pair may have been responsible for more than 70 killings, a claim which if true would rank the duo among the most prolific serial killers in U.S. history.

Crude maps drawn by Shermantine after more than a decade of silence on the fate and whereabouts of his victims helped lead authorities last February to the skeletal remains of five teen girls and young women, including 16-year-old JoAn Hobson, who disappeared in 1985. No one was ever charged with her murder.

Hobson's sister, Michelle Loftis, told Reuters on Friday she would continue to press authorities to search for additional remains. Continued...