Jennifer and Sarah Hart had adopted all six children and recently lived in Woodland, Washington.
Only five bodies were recovered following the crash, though.
They could have also been swept away by seawater because no one in the SUV was wearing a seatbelt, officials said. Two of the Hart daughters remain missing.
An age and ID could not immediately be determined.
An autopsy will be conducted on Tuesday, but authorities warned identifying the body will likely be done through DNA analysis which can take several weeks.
The sheriff's office noted that it is not uncommon after a significant storm that items would surface or wash onto the beach.
"There were no other signs of the other missing Hart children", their statement said.
It's one more in a number of developments in the case this week. California Highway Patrol also released an image of surveillance footage showing one of the mothers, Jennifer Hart, buying groceries the day before the crash.
Mendocino County Sheriff Tom Allman has described the crash as "a crime". "I'm calling it a crime". There, the couple's neighbors recalled unusual encounters with the Hart children.
The family's 2003 GMC Yukon LX was found at the bottom of the cliff in nearby Westport, California, two days later.
In 2010, Sarah Hart pleaded guilty to misdemeanor domestic assault involving one of the children, according to Douglas County court records.
Last week investigators said the crash may have been "an intentional act", noting that information from the car's software and a lack of brake or skid marks suggests that the van stopped at a dirt lookout and then appears to have been driven directly off the cliff.