(Reuters) - Police on Monday searched the ocean off California and highways for three children missing since the sport-utility vehicle carrying their large family plunged off a cliff, killing at least six people in what may have been an intentional crash.

The family’s wrecked GMC Yukon was discovered last Monday below the scenic California coastal cliff, only three days after child welfare authorities in their home state of Washington opened an investigation into allegations the children were being neglected, police and social services said.

According to court records, the family had troubles in the past when one of its two white mothers was charged with domestic assault against one of their six adopted black children.

The Hart family gained attention in 2014 after one of the children, Devonte, was photographed crying in the arms of a white police officer at a protest after the fatal police shooting of an unarmed black teenager in Ferguson, Missouri.

Driver Jennifer Hart, 38, and Sarah Hart, 38, a married couple from Washington state, were killed. So were at least three of their six adopted children, Markis, 19, Jeremiah, 14, and Abigail, 14, police said.

The bodies of their three other children, Hannah, 16, Devonte, 15, and Sierra, 12, were not recovered, but officials have said they believed all the children were in the vehicle and the other bodies may have been washed into the ocean.

“The search for the additional three children belonging to the family continues both at the scene, through water rescue and recovery, and other locations including routes of travel,” the California Highway Patrol said in a statement. “This situation may have been an intentional act and not the result of a traffic collision.”

The SUV appears to have stopped at a dirt pull-off area about 70 feet (21 meters) from the cliff at the edge of California’s coastal Highway 1 in Mendocino County, about 180 miles (290 km) north of San Francisco, according to the vehicle’s on-board computer.

It then accelerated over the edge, leaving no skid marks or other indications of a collision, the statement said. The SUV fell into rocks and was found partially submerged about 100 feet (30 meters) below the highway.

Police searched the family’s home in Woodland, Washington, about 500 miles (805 km) north of the crash site, but found no suicide note.

The Washington State Department of Social and Health Services said it opened an investigation into the family for potential child neglect or abuse after a complaint on March 23.

Neighbors Dana and Bruce DeKalb told the Oregonian newspaper that Devonte had asked them for food for himself and his siblings after his parents withheld it as punishment. The DeKalbs did not respond to requests for comment.

Sarah Hart was convicted of misdemeanor domestic assault of Abigail in 2011 in Minnesota. Court documents show that one of the children, Abigail, told a teacher: “Mom hit me.”