Police had previously said they believed the Hart family died in a suicide plunge from a scenic overlook.
Police also said the Federal Bureau of Investigation was sending its Behavioral Analysis Unit to study cellphone data recovered from one of the family members' phones showing the Hart family's travel information.
The family members were identified as Sandeep Thottapilly, 41; Soumya Thottapilly, 38; Siddhant Thottapilly, 12; and Saachi Thottapilly, 9.
It has not been determined if the family lived in Colorado County at the time of the adoptions.
The family of four - two parents and two children - was traveling from Portland, Oregon, to their home in Santa Clarita, California, last week when they stopped communicating with relatives.
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The vehicle is reported to have submerged in the river around 1:10 pm on April 6. "Several items have been positively identified, by the family members, as belonging to the Thottapilly family". A powerful storm late last week dropped 2 to 5 inches of rain in the region.
Sarah and Jennifer Hart and their six adopted children were believed to be in the family's SUV when it plunged off a cliff in Mendocino County, more than 160 miles (250 kilometers) north of San Francisco.
Siblings Markis, Abigail, and Hannah Hart were adopted by Jennifer and Sarah Hart in Colorado County in 2006, a Texas Department of Family and Protective Services spokesperson confirmed Wednesday morning.
Five bodies were found March 26 near the small city of Mendocino, a few days after Washington state authorities began investigating the Harts for possible child neglect, but three of their children were not immediately recovered from the scene.
A couple vacationing reportedly saw the body and pulled it from the surf before calling authorities.