CORONADO, Calif. — Two fatal tragedies in a week at a century-old oceanfront mansion have created a stunning summer mystery for residents and tourists in this rich beach town where serious crime is rarely a visitor.

“Coronado is that small town that a lot of people remember as a child,” says Sharyn Crown, a longtime local real-estate broker. “But it’s a very sophisticated town.”

This resort community of 25,000 people across the bay from San Diego swells in the summer with tens of thousands of tourists and part-time residents, many of them Arizonans escaping the desert heat at the closest U.S. beach.

Among them is Jonah Shacknai, who struck it rich after founding a cosmetic pharmaceuticals company in Arizona and four years ago paid $12.75million for a historic property here facing the Pacific Ocean. Known as the Spreckels Mansion, his home — more than 12,700 square feet with 12 bedrooms and 10 baths — was the scene last week of twin tragedies that claimed the lives of his girlfriend and 6-year-old son, Max.

Rebecca Zahau, 32, an ophthalmology technician, was found early last Wednesday hanging from a balcony in the home. Sgt. Roy Frank of the San Diego Sheriff’s Department’s homicide unit said she was naked, her hands and feet were bound, and her neck was in a makeshift noose.

Two days earlier, Max was rushed to the hospital after suffering what the family said was a fall on a mansion staircase. The child’s death was announced Sunday.

Max’s death has been treated as an accident, but Frank said Zahau’s death remains a mystery.

“We know it’s not a natural death. We don’t believe it’s accidental,” he said. “Now it comes down to suicide or homicide. Sometimes suicide and homicide can look the same at first glance.”

Results of Zahau’s autopsy have been sealed. Frank said investigators are withholding details of the scene while they pursue the case. He declined to say what was used to bind her hands and feet, but he said her hands were behind her back.

“I think this case essentially is going to be solved by forensics,” Frank said. “We’re hopeful that evidence will tell a story and solve this mystery.”

Frank said only Zahau and Jonah Shacknai’s younger brother Adam were in the house at the time of her death. He said Adam Shacknai cut down Zahau’s body, attempted to revive her and called Coronado police, who brought in the sheriff’s department to handle the case.

The initial call came to police about 6:45 a.m., but a time of death has not been determined.

Jonah Shacknai was in San Diego County when her body was found, Frank said. “We’re not disclosing where he was staying,” he added.

Records detail stormy marriage

Shacknai, 54, is founder and CEO of Medicis Pharmaceutical of Scottsdale, Ariz. He was divorced from his second wife, Dina, in January, after repeated stormy encounters.

Police in Paradise Valley, Ariz., released reports Monday detailing domestic incidents. In 2008, Shacknai reported that his wife had tried to choke him. His wife told police she was bitten by the family’s German shepherd during the incident, and there had been other similar cases. In 2009, she called police and reported he elbowed her in the chest during a quarrel. He denied hurting his wife and alleged she attacked him. No arrests were made.

The couple released a statement Monday saying of Max’s death, “His loss is unimaginable.”

The Arizona Republic reported that after they separated, Dina Shacknai kept a home of her own at the Coronado estate, and Jonah Shacknai lived in the main mansion with Zahau.

A shock to a ‘lovely’ town

Scores of beachgoers walk by the mansion daily, a short walk from the fabled Hotel del Coronado. In the mansion’s front yard rests a small shrine of flowers and Zahau’s picture. Except for the occasional TV news truck out front, beachgoers paid little attention to the home, though many said they were following the case.

Joe and Lisa Suttell of Phoenix, who brought their two children to the beach across Ocean Boulevard from the mansion, said Coronado is hugely popular with Arizona residents, and the twin tragedies won’t diminish vistors’ enthusiasm for the quiet, if traffic-clogged, resort town.

“It’s just a very relaxing, kid-friendly kind of place,” Joe Suttell said.

Mary Ann Voevodsky of Prescott, Ariz., said she and her husband Steve and their daughter Hanna have been coming here every summer or fall since 2001. “You never think of something like this happening in a community like this,” she said.

Tex Lawrence, 85, a retired Marine colonel, said the news was a shock to his “lovely” town.

“It’s going to make me sick,” he said.

Younger residents were fascinated by the mystery.

“It’s like nothing ever happens here, so it’s a big deal,” said Margarita Navarro, a high school junior. “It’s really scary.”

Crown, the real estate broker, said the community respects privacy. Shacknai’s purchase of the 1908 mansion scarcely attracted notice, she said.

“It’s a very strange story for any town, but particularly this town,” she said. “Things like this don’t really happen here.”