SIX children and their parents are dead after their vehicle plunged off a cliff in a mysterious case that investigators believe was not deliberate.

The married couple have been identified as Jennifer and Sarah Hart and reportedly live in Portland, Oregon's largest city. Three of the children were reportedly adopted.

Investigators say Jennifer Hart drove a 2003 GMC Yukon SUV into a large dirt turnout and then for unknown reasons, drove directly off a cliff. The car plunged 30 metres, overturned, landing on its roof onto a boulder on the beach and partially submerging itself in the surf.

The mothers were wearing seatbelts but the children were not, accoridng to The Press Democrat.

Mendocino County Sheriff Tom Allmon said a passer-by called 911 after the crash at approximately 3.40pm local time but investigators are unable to pinpoint the exact time of the incident.

"An entire family vanished and perished during this tragedy," he said, according to the Associated Press.

One of the missing teenagers is Devonte Hart, who gained fame after a photo of him hugging a police officer went viral after the 2014 protests over the shooting death of Michael Brown, ABC News reports. He was pictured holding a "Free Hugs" sign.

Yet a couple who lived next door to the family say they called authorities over fears Devonte was "going hungry"

Bruce and Dana DeKalb "called child welfare officials Friday after becoming concerned Devonte Hart had been coming to their house too often in the past week asking for food," AP reports.

The same couple reported one the female children ringing their doorbell at 1.30am "in a blanket saying we needed to protect her. She said that they were abusing her".

Searchers have been scouring the Northern California coast for three of the children after the family's SUV drove off the Pacific Coast Highway overnight, plunging into rocks below the ocean cliff. Boats, aircraft and drones are being used to find the missing.

Sheriff Allmon said he believes all six of the children were in the vehicle when it plunged off the cliff, despite only three of the bodies of the children having been discovered.

Investigators have been scanning the area for clues while Sheriff Allmon says the case is curious because there were no skid or brake marks, despite the fact there is no reason to believe it was a deliberate act.

The dead children were identified as Markis Hart, 19, Jeremiah Hart, 14, and Abigail Hart, 14.

Searchers were looking for Hannah Hart, 16, Devonte Hart, 15 and Sierra Hart, 12.

Clark County sheriff Sgt. Brent Waddell told the Associated Press the Harts had recently received a visit from Child Protective Service for a welfare check, when the sheriff's office entered the house, they found no obvious signs of violence.

Online records show Sarah Hart pleaded guilty to a domestic assault charge in Minnesota in 2011. It led to the dismissal over a charge of malicious punishment to a child.

Drugs or alcohol are not considered to be factors in the crash.

More to come