A woman who drove off a Northern California cliff last month in an SUV carrying her wife and children was drunk, authorities said Friday.

Toxicology tests found Jennifer Hart had an alcohol level of 0.102, said California Patrol Capt. Bruce Carpenter. California drivers are considered drunk with a level of 0.08 or higher.

Toxicology tests also found that her wife Sarah Hart and two of their children had "a significant amount" of an ingredient commonly found in the allergy drug Benadryl, which can make people sleepy. Toxicology results for a third child killed are still pending, Carpenter said.

None of the vehicle's occupants were wearing seatbelts, he said.

This March 2016 photo shows the Hart family of Woodland, Wash., at a Bernie Sanders rally in Vancouver, Wash. Authorities in Northern California have previously said they believe all six children from the family were in a vehicle that plunged off a cliff. (Tristan Fortsch/KATU News/Associated Press)

Sarah and Jennifer Hart and their six adopted children were believed to be in the family's SUV when it plunged off a cliff in Mendocino County, more than 250 kilometres north of San Francisco.

Authorities have said that data from the vehicle's software suggests the crash was deliberate, though the California Highway Patrol has not concluded why the vehicle went off an ocean overlook on a rugged part of coastline. A specialized team of accident investigators is trying to figure that out with help from the FBI, Carpenter said.

Five bodies were found March 26 near the small city of Mendocino, a few days after Washington state authorities began investigating the Harts for possible child neglect, but three of their children were not found at the scene.

Two are missing and a body has been found but not identified.

The 31-metre drop killed the women, both 39, and their children Markis, 19, Jeremiah, 14, and Abigail, 14. Hannah, 16, Devonte, 15, and Sierra, 12, have not been found.