Three San Francisco couples set out Monday for their annual road trip to Ashland, Ore., for the town’s famous Shakespeare festival. They drove separately and planned to meet at 6:30 p.m. on the terrace of their favorite Japanese restaurant there.

They had booked a table for six, but only four showed up for dinner.

Judith and Wylie Sheldon were found dead in their running car on the side of the road to Oregon, shocking their friends and family and leaving a hole in San Francisco’s arts and film world.

Ms. Sheldon, 84, was the daughter of William Wyler — who won three Oscars for best director — and chaired the board of the San Francisco Silent Film Festival. Mr. Sheldon, 86, was a prominent lawyer.

David Smith, who had befriended the couple more than 40 years ago, said in an interview that he and the others at the dinner table had grown nervous as time ticked on and their friends did not answer repeated calls to their cellphones. They learned they had not checked into their hotel either.

The friends eventually learned from one of the couple’s sons that the California Highway Patrol had found the couple at 5:46 p.m., both dead inside their running Jeep Compass. It was parked on the side of Interstate 5, north of Redding, Calif., more than 100 miles from their destination, the authorities said. Ms. Sheldon was driving, while Mr. Sheldon was in the passenger seat, according to the authorities.

The Redding area on Monday was under an extreme heat warning issued by the National Weather Service. Temperatures reached 109 degrees, according to the Weather Service.

Mr. Smith said he learned from the son that the couple had been found without any water or other liquids in the car. The fan was on high, but the air conditioning was not working, meaning they might have been blasted with hot air, Mr. Smith said. The windows were rolled down. The car had plenty of gas, and there were no signs of mechanical failure or foul play, Mr. Smith said the son told him.

“They didn’t crash. They stopped. They both just died there,” Mr. Smith said. “The entire thing is so bizarre. We’re still in a state of shock.”

The circumstances and cause of the couple’s death is under investigation but “appears to be medically related,” the Highway Patrol said in a statement.

Whether the heat contributed to the couple’s death “may be determined” by an autopsy, a spokesman for the Shasta County Sheriff’s Office said, adding that one had not been scheduled yet and could take several weeks to complete.

“We’ll just have to see,” the spokesman, Tim Mapes, said.

The Sheldons met at Stanford University and had two sons. They lived in a large home in San Francisco’s upscale Pacific Heights neighborhood that had views of the bay from the front and a garden out back.

They hosted many parties there on behalf of the San Francisco Silent Film Festival and sometimes let revelers pose for photos with Mr. Wyler’s Oscar statuettes. Ms. Sheldon fell in love with silent movies after first seeing those created by her father — before his better known blockbusters like “Ben-Hur” and “Roman Holiday” — only about 30 years ago, said Anita Monga, artistic director of the festival.

Stacey Wisnia, the festival’s executive director, said the couple was generous, delightful and unassuming.

Back in Ashland, Ore., Mr. Smith said the four remaining friends had distracted themselves from their grief by attending plays, including “A Midsummer Night’s Dream” and “Come From Away.” They were able to give away their friends’ tickets.

Ms. Monga had last seen Ms. Sheldon just last month at the film festival, which was held at the newly remade Castro Theater.

“This is such a shock,” Ms. Monga said of the deaths. “Also because it’s still a mystery.”