FRESNO, Calif. -- The sheriff of the remote region where Charles Manson hid after a killing spree in the summer of 1969 said Friday that he will allow researchers to begin digging into the sandy soil in search of possible human remains.

In February, a team of forensic researchers from Tennessee's Oak Ridge National Laboratory visited the Death Valley ranch where Manson took refuge and found at least two sites that could be clandestine graves holding the bodies of additional victims.

Inyo County Sheriff Bill Lutze said he will allow a limited four-day excavation at Barker Ranch beginning May 20 because forensic tests of the soil had produced mixed results.

"There was no consistent response from the dogs that searched, and no conclusive findings from the soil samplings tested by top experts in the field," Lutze said in a statement. "The only way to determine once and for all whether there are bodies buried at Barker Ranch from the time of the Manson family is to proceed with limited excavation."

National Park Service officials said Thursday that the ranch, which lies within the boundaries of Death Valley National Park in the rugged Panamint Mountain range, would be closed for as long as four days "to protect the integrity of the investigative process."

The ranch was briefly closed in April, but a decision to dig was put off because a piece of high-tech soil-testing equipment was damaged en route to the site. Authorities said they wanted to conduct tests using lasers before disturbing the soil.

Scientists completed four kinds of tests at five locations on the ranch and found some corroboration of a "possible find of human remains" at some of the sites, Lutze said.

Manson and his followers hid out in the decrepit ranch after a series of gruesome murders that set Los Angeles on edge in 1969. They were arrested there in a raid.

Manson is serving a life sentence at Corcoran State Prison for the murders of seven people, including actress Sharon Tate.