Decades after law enforcement raided the ranch where Charles Manson hid following a 1969 killing spree, detectives and scientists returned to hunt for undiscovered graves Tuesday.

A group of about 20 people, including scientists and law enforcement personnel, headed to the secluded ranch within Death Valley National Park early in the morning and started digging. The team includes specialists in detecting disturbed soils and chemical markers that indicate likely grave sites.

"It's going to be back-breaking manual labor," said Carma Roper, a spokeswoman for the Inyo County Sheriff's Department.

The expedition to the secluded ranch in the rugged Panamint Mountain range is expected to last through Thursday. Temperatures in the park are forecast to surpass 100 degrees.

For years, rumors have swirled about other possible Manson victims _ hitchhikers who visited the ranch and were not seen again, and runaways who drifted into the camp, then fell out of favor.

The decision to further investigate the site where Manson and his followers hid following the murders of actress Sharon Tate and six others came after initial tests in February found at least two sites that could be graves.

A group including two national lab researchers, a police investigator with a cadaver-sniffing dog, and an anthropologist with a magnetic resonance reader determined there was enough evidence to contact the Inyo County Sheriff's Department.

After further soil sampling with inconsistent results, Sheriff Bill Lutze agreed to the exploratory excavation. The National Park Service has closed the ranch to the public for the duration of the dig.

The searchers will use technology that wasn't available when Manson and his followers were arrested nearly 40 years ago, such as radar, magnetometers and portable gas-chromatograph and mass spectrometers that can detect chemical markers characteristic of bodies in decomposition. They will also dig with shovels, Lutze said.

Manson is serving a life sentence at Corcoran State Prison.