Interviews with relatives and friends suggest all six of the family's children were inside the sports utility vehicle when it plunged off a coastal cliff last week, police said in a statement.
The bodies of driver Jennifer Hart, 38, Sarah Hart, 38, a married white couple, and three of their African-American adopted children were recovered after the March 26 crash.
Police had been searching for the missing children along the coastal highway the family took, but are now more certain than ever that their bodies were swept into the ocean after their GMC Yukon fell 100 feet (30 meters) onto seashore rocks.
The crash occurred three days after child welfare services opened an investigation into allegations of potential abuse or neglect of the children at their home in Woodland, Washington.
"Information obtained through interviews with family and friends indicated it was rare for the family to be apart, especially while travelling," Mendocino County Sheriff's office said in a statement.
There were no tire marks that may have suggested rapid acceleration or erratic movements, indicating the vehicle was steadily driven from a standstill towards the edge, California Highway Patrol spokesman Cal Robertson said on Thursday.
Police have yet to ascertain whether a struggle took place between Hart and her partner or whether all the children were alive at the time of the incident, Allman told CNN on Wednesday.
The missing children include 15-year-old Devonte Hart, who gained notoriety in 2014 after being photographed hugging a white police officer at a protest after the fatal police shooting of a young black teenager in Ferguson, Missouri.
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