Two women were killed and a third injured after their car crashed into a tree on private land belonging to the Kennedy family Tuesday afternoon.

The driver, 70-year-old Judith Morse from West Tisbury, and front-seat passenger Susan S. Lombard, 69, of Lyman, NH, were pronounced dead at the Martha’s Vineyard Hospital, according to the Massachusetts state police. The rear-seat passenger, Susan Papanicolaou, 68 of Minneapolis, sustained minor injuries and was treated at the hospital.

State police sgt. Thomas Medeiros said the women were driving on Red Gate Farm Road — part of the vast Kennedy compound off Moshup Trail — when they were spotted by the property’s caretaker, Albert Fischer. Mr. Fischer turned away and lost sight of the vehicle. He drove out to the main road, Moshup Trail, looking for the car, but did not find it, said Sgt. Medeiros.

Soon after returning to the property, Mr. Fischer noticed that an electrical box on the property had been tipped over. He found the women’s car resting at the bottom of a hill and called police for help.

Sgt. Medeiros said investigators believe Ms. Morse was backing up when she lightly hit a tree. “At that point she apparently put the car in drive and we believe she just accelerated from there,” he said. “She struck an electrical box, a porch, then she kind of went up a hill and struck a tree. It was just a perfect storm.”

“It’s one of those freak accidents,” said Chief Belain. “It’s bizarre.”

Mr. Belain said he did not know why Ms. Morse accelerated, whether due to nervousness or, perhaps, a medical emergency.

The Aquinnah police and fire departments, Sgt. Medeiros, tribal rangers, Tri-Town Ambulance, Chilmark fire fighters and paramedics from each of the towns responded to the call at 12:06 p.m. Dispatchers alerted the hospital to prepare for two priority-one patients, a code that indicates the patient is critically ill or has sustained injuries that require immediate attention. A state police accident reconstruction team and crime services unit arrived later in the day.

Sgt. Medeiros said the accident remains under investigation, “but just because we haven’t put it all together yet.”

In a statement released by the John F. Kennedy Library Foundation, Caroline Kennedy and Edwin Schlossberg said, “Our thoughts and prayers are with the families of the victims of this tragic accident.”

Jacqueline Kennedy Onassis purchased the Red Gate Farm estate in Aquinnah in 1978. The property covers 366 acres between Moshup Trail and Squibnocket Pond.

The Martha’s Vineyard Museum also released a statement to board members on Wednesday, noting that Ms. Morse was the mother of Chris Morse, a longtime board member and owner of the Granary and Field Galleries in West Tisbury and the North Water Gallery in Edgartown. “Our sympathies go out to the Morse family at this moment of loss,” David Nathans, executive director of the museum, said.

A memorial service for Ms. Morse will be held at a later date to be announced, according to Chapman, Cole & Gleason Funeral Home in Oak Bluffs, which is handling the arrangements.