Authorities have unearthed more skull fragments and other human remains, along with clothes, a purse and jewelry, in a well in rural Northern California, an area where a convicted serial killer said there may be 10 or more victims.

Information provided by convicted serial killer Wesley Shermantine has led to the discovery of human remains in a well in Northern California. (California Department of Corrections/File)

The remains and other items were found 13 metres deep in the well on an abandoned cattle ranch, on Sunday. San Joaquin County Deputy Sheriff Les Garcia said in a statement.

After two days of searching the site, investigators, public works employees and volunteers have found more than 300 human bones, Garcia said. The search for more human remains was suspended because of rain on Monday.

"We are bringing the dirt and debris up using excavators and we're searching piles. If it's raining, we will wait," Garcia said before dawn.

Sunday marked the fourth straight day that remains have been found with the help of a map prepared by death row inmate Wesley Shermantine. He and childhood friend Loren Herzog became known as the "Speed Freak Killers" for a methamphetamine-fuelled string of killings.

Skull fragment sent for DNA testing

A piece of a human skull and bones found Saturday at the ranch will be sent to the Department of Justice in the hopes of identifying them through DNA testing, Garcia said. Dental records identified remains found Thursday in Calaveras County as those of Cyndi Vanderheiden, 25, who disappeared in 1988.

Another set of remains were found Friday in the same area, and the parents of 16-year-old Chevelle "Chevy" Wheeler, who disappeared in 1985, said authorities told them that's where their daughter was believed to be buried.

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

Herzog was paroled in 2010 to a trailer outside the High Desert State Prison in Susanville. He committed suicide outside that trailer last month after Sacramento bounty hunter Leonard Padilla told him Shermantine was disclosing the location of the well along with two other locations.

Crews are expected to be searching the ranch in Linden for several days, at what Garcia has said would be a "slow and tedious" pace. The property, south of Sacramento, was once owned by Shermantine's family.