In the three days since a passerby happened to notice an SUV upside-down at the bottom of a cliff along the California coast, investigators have determined that five members of the same family were killed in the crash.

But three of the family's children are missing, and investigators say it's not even certain they were in the SUV at the time it tumbled 100 feet into the sea.

Among the missing is a teenage boy who drew national headlines when he appeared in a touching photo at a protest a few years ago.

Nobody reported seeing the crash, so authorities don't know exactly when or how it happened.

And at the family's home in Washington state, hundreds of miles to the north, neighbors say the socially minded family of eight -- two mothers with six adopted children -- had raised their suspicions.

Overturned in the water

The bodies of Jennifer and Sarah Hart, both 38, were found Monday inside their overturned SUV near a remote stretch of Highway 1 in Northern California, the Mendocino County Sheriff's Office said.

The Mendocino County Sheriff's Office says the bodies of Jennifer and Sarah Hart and three of their children were found in and around the vehicle. Their other three children are still missing.

Outside the car, emergency responders found the bodies of Markis Hart, 19, and Jeremiah Hart and Abigail Hart, both 14. They found scattered belongings, including a purse, nearby.

The vehicle had plunged into the water from a flat, gravel-covered outcrop where travelers can stop to admire the view, walk their dog or stretch their legs, the sheriff said. Investigators found no skid marks or brake marks, and Sheriff Tom Allman said it's not clear yet whether the SUV rolled or launched off the edge.

What is known is that three of the Hart children are missing: Hannah, 16; Devonte, 15; and Sierra, 12. The sheriff said there is "every indication" that they were riding in the SUV, too, but it's possible they were left behind with friends.

It was Devonte who appeared in the viral photograph from 2014, which was taken at an Oregon protest against police brutality after the Ferguson, Missouri, police shooting. Devonte, who is black, was shown hugging a white police officer as tears streamed down the then-12-year-old's face.

Devonte Hart, then 12, hugs Sgt. Bret Barnum at a rally in Portland, Oregon, in 2014.

Neighbors' suspicions

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, said Sgt. Brent Waddell. He said the office is working with California authorities to find the missing children.

The Harts' neighbor, Bruce DeKalb, said the family moved in about 10 months ago and was "very private." He said he rarely saw the children playing outside, even in the warmer months.

Another neighbor, Bill Groener, said the children were home-schooled and kept inside most of the time.

DeKalb told HLN on Thursday about upsetting encounters he had with two of the Hart children.

"One of the girls came to the door at 1:30 in the morning and said that she needed help and the parents were not treating her properly, and (she) wanted us to protect her," DeKalb said. "We ended up getting her back to her parents ... and then I went over there the next morning and just checked on things, and everything seemed normal, and we let it go from there."

DeKalb said he and his wife also encountered Devonte, the one child he saw doing outside chores, such as taking the garbage bins to and from the street.

"About a week ago, he (Devonte) started coming over asking for food and saying that they were taking meals away from him due to punishment," DeKalb told HLN. "And it started out as one time a day and escalated up to three times a day, until a week went by and we decided that we needed to get professional help."

Last Friday, DeKalb said, he called Child Protective Services. They arrived at the house just after Jennifer Hart came home from work, he said, and she didn't answer the door. Sarah Hart came home not long afterward, he said, and by Saturday morning the family and their vehicle were gone.

Different view of the couple

After the Friday visit, Child Protective Services tried to visit the family twice more, on Monday and Tuesday, to no avail, according to the Washington State Department of Social and Health Services. The agency was trying to get in touch after the "now deceased children were identified as potential victims of alleged abuse or neglect," according to a statement from the Washington State Department of Social and Health Services.

The department said it had no history with the family.

Before the Harts moved to Washington, they had lived in Minnesota, where Sarah Hart pleaded guilty to misdemeanor domestic assault involving one of the children in 2010, according to court records from Douglas County, Minnesota.

The child had told an elementary school teacher of pain on her back and stomach, which was discovered to be bruises. Hart told police she had spanked the child over the edge of a bathtub because of behavioral issues. She was sentenced to community service and one year of probation.

Friends of the Harts had a different view of the couple, describing them to CNN affiliate KOIN as loving parents who gave the kids everything they had and radiated positivity.

"Jen and Sarah really were the kind of parents that I think the world desperately needs," friend Zippy Lomax told KOIN.

When deputies gained entry to the family's home Tuesday for the welfare check, they found conditions that indicated any trip would have been temporary, Allman said. They found belongings, a pet and some chickens.

Though authorities don't know when the SUV went off the cliff, Allman has pleaded for information about the family's whereabouts in the hours leading up to the crash. He said there is no reason to believe the wreck was an intentional act.

Lomax told The Oregonian newspaper that the Hart family enjoyed traveling together, taking scenic routes to new places.

"I'm sure they were going somewhere special and fun," she told the newspaper, "as they often did."