He told NBC News that California authorities were "tirelessly searching for the missing children along the coastline", while authorities in California and Washington were "conducting interviews and attempting to establish a timeline and routes of travel in an effort to rule out any other possibilities".

The family's SUV appears to have been stopped by the side of the road in Mendocino County, about 180 miles (290 kms) north of San Francisco, before accelerating to as fast as 90 mph (145 kph) and hurtling off a cliff, acting Assistant CHP Chief Greg Baarts told reporters on Sunday.

Bodies of five family members have been recovered. Three children were confirmed to have died in the crash.

Baarts stressed that investigators were still verifying the information, but that authorities believed that a felony had been committed in the crash.

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.

"To the best of my knowledge, there was not a suicide note found at the residence", said Baarts. "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".

"Preliminary investigation indicates to us that this incident might have been an intentional act and not the result of a traffic collision", Captain Baarts said. One of the mothers, Sara, had been charged with a misdemeanor offense of domestic assault in 2011.

The two women, both 38, were found dead inside the SUV, while three of their children - Markis Hart, 19, Jeremiah Hart, 14, and Abigail Hart, 14 - were discovered outside the vehicle.

The family gained some level of fame when a photo taken during a 2014 Portland protest of Devonte Hart hugging a police officer with tears streaming down his face went viral. Their other kids, Hannah, 16, Devonte, 15, and Sierra, 12, are still missing and feared dead. Devonte was holding a "Free Hugs" sign.

Neighbors reported concern for the couple's children, including a belief that the parents may have been withholding food.

A neighbor Bruce Dekalb said, "I think that it's a little odd that three of them left the vehicle and three of them didn't".

The boy, who like his siblings was being home-schooled, had asked that the couple leave food in a box by a fence for him, the couple told CBS News.

Deputies from the Clark County Sheriff's Office in Washington executed a search warrant Thursday night at the family's home in Woodland, looking for any evidence in the missing persons case, Sgt. Brent Waddell told media late last week. Authorities said the family left OR to escape intense scrutiny that began in 2014, when Devonte Hart was photographed sobbing in the arms of a white police officer in Portland, where people had gathered to support protests in Ferguson, Mo.

"There have been red flags", Baart told USA Today on Monday.

Family friend Max Ribner took issue with the notion it was something other than a awful accident.

Allman said there was "every indication" all six of the couple's children were in the vehicle.

"They transformed these kids lives", he told USA media.