More than 300 human bones have been unearthed from a well in rural California, where a convicted serial killer told authorities there may be 10 or more victims from a killing spree in the 1980s and 1990s.

The remains were found in just two days of searching at an abandoned cattle ranch in Linden, approximately 60 miles east of San Francisco. Rain could now slow the search, police said.

The San Joaquin county sheriff's spokesman, deputy Les Garcia, said: "We are bringing the dirt and debris up using excavators and we're searching piles. If it's raining, we will wait."

The remains were found with the help of a map prepared by a death row inmate, Wesley Shermantine. He and a childhood friend Loren Herzog became known as the Speed Freak Killers for a methamphetamine-fueled killing spree that claimed as many as 15 victims from the 1980s until their arrests in 1999.

Shermantine was convicted of four murders and sentenced to death, but remains in prison. Herzog was convicted of three murders and sentenced to 77 years to life in prison, though that was reduced to 14 years. An appeals court quashed his first-degree murder convictions after ruling his confession was illegally obtained.