(CNN) -- Authorities said Friday they will begin digging for evidence of graves at a California ranch that once was home to the infamous "family" of convicted killer Charles Manson.
Soil testing to determine the possible presence of buried bodies was inconclusive, authorities said Friday, so "exploratory excavation" will be done later this month, Inyo County Sheriff Bill Lutze said.
The dig will be carried out "to either confirm or put to rest speculation that bodies may remain there from the Charles Manson era," Lutze said in a written statement.
In March, search crews found indications that human remains may lie within a few yards of Barker Ranch, the onetime hideout of the "Manson family."
The sites were first identified by trained dogs, and testing equipment from the Oak Ridge National Laboratory also indicated the possible presence of remains, said Sgt. Paul Dostie of the Mammoth Lakes, California, police department.
The excavation will take place at sites where cadaver dogs gave mixed indications and soil testing was inconclusive, Lutze said.
"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," he said. "Therefore, I believe 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 in a very few areas."
He said four methods of testing were used at five sites on the ranch. No site was identified by all four methods as possibly having remains, but positive findings by one method were corroborated by another at some of the sites, the sheriff's department said.
The excavations will begin May 20 and will continue through about May 23, although additional days will be added if needed, he said.
Barker Ranch is National Park Service property, and will be closed to the public during the excavation, the sheriff's department said. Media access will be limited, but daily briefings will be provided.
After Dostie's dog and another dog, from NecroSearch, a nonprofit organization that specializes in finding clandestine graves, indicated the possible presence of human remains, testing equipment from Tennessee's Oak Ridge laboratory indicated two likely grave sites and a third possible grave site, Dostie said.
Manson and four others were convicted of murder and other charges in connection with a two-night spree in August 1969 that included the slaying of actress Sharon Tate. The slayings -- and Manson and his family members -- transfixed the nation.
Manson is serving a life sentence at California's Corcoran State Prison. CNN sent letters to him and to codefendant "Tex" Watson, asking if there were victims buried behind Barker Ranch.
Manson did not reply, but Watson did, writing in a letter:
"I was the first family member to go to the desert after the murders and also the first to leave. I say this only to let you know that no one was killed while I was in the desert. But I don't know what took place after I left. I don't think there were any more killed, I hope not! I have absolutely nothing to hide."