The SUV that authorities said was carrying a family of eight - two parents and six adopted children - was traveling at 90 miles per hour before it plunged off a California cliff last week, according to court documents obtained by a local TV station.
Devonte, 15, Hannah, 16, and 12-year-old Sierra are still missing, but presumed dead.
"This specific location is very hard to search because the ocean currents and tides are strong, it's unpredictable, and the murkiness of the water makes it hard to see", Captain Greg Van Patten of the Medocino County Sheriff's office told the Associated Press (AP) news agency.
Their three other adopted children were believed to be in the auto, according to deputies, but they have not been located.
Two weeks ago, Bruce and Dana DeKalb, next door neighbours of the Harts, called state child protective services because Devonte had been coming over to their house nearly every day for a week, asking for food.
Known as the Hart Tribe, the multiracial family of two married women - Sarah and Jennifer Hart - and six adopted children often took spontaneous road trips to camp and hike and traveled to festivals and other events, offering hugs and promoting unity.
Bodies of five family members have been recovered. CPS confirmed that they had opened an investigation, which identified the six adopted siblings as "potential victims of alleged abuse and neglect".
The child told police that Jennifer Hart hit her. "When asked what she had done to make her mom mad, (the child) said that she had a penny in her pocket and that this made her mom mad".
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The family gained attention after one of the children, Devonte, was photographed crying in the arms of a white police officer at a 2014 protest just months after the fatal police shooting of an unarmed black teenager in Ferguson, Missouri. One of the mothers, Sara, had been charged with a misdemeanor offense of domestic assault in 2011.
Two weeks ago, Bruce and Dana DeKalb, next-door neighbors of the Harts in Woodland, Wash., called state Child Protective Services because Devonte had been coming over to their house nearly every day for a week, asking for food.
A neighbor Bruce Dekalb said, "I think that it's a little unusual that three of them left the vehicle and three of them didn't". He said his parents would withhold food as punishment. The agency had no prior history with the family, said Norah West, a spokesperson with the Washington Department of Social and Health Services.
The Clark County Sheriff's Office obtained a searched warrant Thursday to retrieve items like computers, credit cards and bank statements to help determine a probable cause.
Investigators are looking into "red flags" in a Washington state family's past in hopes of explaining why their SUV went off a 100-foot cliff in an apparent suicide plunge.
Van Patten said he was not aware of any other evidence of abuse.
Friends of the Harts had a different view of the couple, describing them to CNN affiliate KOIN as loving parents who gave the kids everything they had and radiated positivity.
And in 2013, social service authorities in OR, where the Harts lived at the time, contacted the West Linn Police Department about them, police said Monday.