Image copyright Tristan Fortsch / KATU News Image caption Jennifer and Sarah Hart and their six children at a Bernie Sanders rally in Vancouver, Washington in 2016

A family of eight who died after their car plunged off a California cliff were killed in a "murder-suicide", the state's coroner's office has ruled.

The Mendocino County Sheriff-Coroner made the statement after a two-day inquest into the tragic 2018 deaths.

The parents had researched suicide online before the crash, officials say.

The crash, which killed six adopted children ages 12 to 19, came days after an inquiry was opened into allegations of child abuse against the parents.

Sheriff-Coroner Thomas Allman explained that a 14-panel jury had ruled unanimously that the two women - Sarah and Jennifer Hart - deliberately drove off a 100ft (30m) cliff with their children in tow.

Investigators say the couple, who had been living in Washington state, had researched methods of suicide on the internet for hours as they drove the family south along the northern California coast.

Image copyright California Highway Patrol Image caption The car carrying the family landed in the Pacific Ocean

Sarah Hart, and several of the children, were found to have Benadryl - an allergy drug that causes drowsiness - in their bodies at the time of the crash.

The driver, Jennifer Hart, was over the legal limit for alcohol at the time, California Highway Patrol investigator Jake Slates said, according to the Associated Press.

"They both decided that this was going to be the end," he said at a coroner's inquest. "That if they can't have their kids that nobody was going to have those kids."

A witness who was camping nearby at the time told the jury that he heard a car engine revving up and screeching out around 03:00 local time (10:00 GMT) on 26 March.

Investigators had suspected the act had been deliberate, based on the car's computer and the lack of braking skidmarks in the road, which would have indicated an accident.

The bodies of five of the children - Markis, 19; Hannah, 16; Jeremiah, 14; Abigail, 14; and Ciera, 12 - were found within or nearby to the vehicle, which landed upside down on a Pacific Ocean beach.

Devonte, 15, is still considered missing but is presumed dead.

Investigators found the that the couple had been accused of child abuse several times, and that days before the crash officials in Washington state had opened an investigation after Devonte rang a neighbours' doorbell and said his parents were withholding food.

In 2010, Sarah Hart admitted to harming her daughter Abigail and was convicted of misdemeanour domestic assault.