(Reuters) - California police on Tuesday tried to retrace the travel route of a Washington state family to help find three children missing since their vehicle plunged off a cliff, killing at least six people in a crash believed to be deliberate.

The family’s sports utility vehicle was discovered at the foot of a California coastal cliff on March 26, three days after child welfare authorities opened an investigation into allegations the two mothers’ six adopted children were being abused or neglected, police and social services officials said.

Driver Jennifer Hart, 38 and Sarah Hart, 38, a married white couple, were killed, together with at least three of their African-American children whose bodies were found.

A surveillance video from a business in Fort Bragg, California, may show Jennifer Hart the day before the crash, the Mendocino County Sheriff’s office said in a statement on Tuesday.

The bodies of the three other children have not been recovered, leading investigators to believe they were washed into the ocean or that the children were not in the vehicle.

“We don’t know for sure either way if the kids were in the vehicle or not,” California Highway Patrol spokesman Cal Robertson said. “We want to do our due diligence and investigate the possibility these children could be elsewhere.”

One of the missing children is 15-year-old Devonte Hart, who drew international attention after he was photographed hugging a white police officer at a rally in Portland, Oregon, months after the shooting of an unarmed young black man in Ferguson, Missouri.

While searchers combed coastline for the children, police worked out the route their GMC Yukon took from their home in Woodland, Washington, around 500 miles (805 km) away.

“We’re reaching out to the public to see if they’ve seen the family, saw the vehicle before the incident,” Robertson said.

The SUV stopped at a scenic Pacific Ocean viewpoint at the edge of Highway 1, about 180 miles (290 km) north of San Francisco, then accelerated over the edge, leaving no skid marks or indications of a collision.

“We don’t have any physical evidence to suggest that this is not an intentional act,” Robertson said.

A large-scale search and rescue operation will begin on Wednesday morning near the crash site involving aircraft, boats and people on land, police said.

The Washington State Department of Social and Health Services said it had been alerted on March 23 of allegations about the treatment of the Hart children and had launched an investigation into potential abuse and neglect.

Devonte had asked neighbors for food for himself and his siblings, saying that his parents withheld it as punishment, the Oregonian newspaper reported.

Sarah Hart in 2011 had been convicted for misdemeanor domestic assault of her daughter Abigail in Minnesota, court documents show.