The speedometer inside a lesbian couple's SUV was 'pinned' at 90 mph after it plunged 100 feet off a California cliff with their six adopted children inside, according to court documents.

Jennifer and Sarah Hart, both 39, were killed when their 2003 GMC Yukon XL crashed off the scenic Pacific Coast Highway in Northern California this week.

Authorities say all eight members of the Hart family, including the six children aged 12 to 19, are presumed dead.  

The bodies of three of the children - Markis Hart, 19, Jeremiah Hart, 14, and Abigail Hart, 14 - were located at the bottom of the cliff.

Police believe the couple's three other children - Hannah Hart, 16, Devonte Hart, 15 and Sierra Hart, 12 - were also in the car at the time but their bodies have not yet been found. 

Mystery surrounds the horror crash given no one saw their SUV drive off a flat, dirt pullout overlooking the Pacific Ocean.  

A search warrant was served at the family's home on Thursday to look for items including a travel itinerary, journals and possible suicide notes.  

According to an affidavit for the warrant, cited by Fox 12 Oregon, the speedometer was 'pinned' at 90 mph.

Investigators said there was no evidence the car hit the embankment as it 'traversed towards the tidal zone below', and there were no 'acceleration marks, tire friction marks or braking furrow marks'.

'Based upon the California Highway Patrol investigation, it is their belief "a felony has been committed,"' the court documents stated, according to Fox 12. 

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