PARIS, OCT. 3 -- Monaco's star-crossed royal family was struck again by tragedy today when Stefano Casiraghi, the 30-year-old Italian financier and husband of Princess Caroline, was killed in a speedboat accident while defending his title at the World Offshore Championships near Monte Carlo.

Casiraghi, known as one of the young lions of the European jet set, and his copilot Patrice Innocenti drove their twin-engine catamaran Pinot di Pinot straight into a wave at nearly 100 mph during a morning heat, eyewitnesses said. The 42-foot boat flipped into the air, ejecting Innocenti while Casiraghi remained strapped in his seat. When the vessel slammed back into the water, Casiraghi was killed instantly by the blow, according to Gianfranco Rossi, head of the race organizing committee. Innocenti was hospitalized with minor injuries.

The death of Caroline's fun-loving but steadfast husband was the latest trauma to afflict the glamorous royal family of Monaco's reigning Prince Rainier, whose storybook romance with Grace Kelly, the Hollywood actress, ended tragically when she died from injuries suffered in a car accident eight years ago. That accident seriously injured Caroline's younger sister, Princess Stephanie, and left the principality's ruler so emotionally depressed that friends say he has never recovered from his wife's death.

Princess Caroline, 33, cut short a visit to Paris when she learned of her husband's death and rushed home in black mourning clothes. Since their marriage in 1983, she has largely concentrated on rearing their three children and assuming Princess Grace's former role as a patron of cultural arts in the Mediterranean principality renowned for high-stakes gambling casinos and celebrity tax exiles.

Her relatively low profile in recent years reflected a dramatic change from the days when she dismayed her parents by dabbling in modeling and marrying the French playboy Philippe Junot, 17 years her senior. Their tempestuous two-year union was chronicled by gossip magazines around the world, invariably depicting him in the company of beautiful women other than his wife. A practicing Roman Catholic, Caroline sought to have the marriage annulled. Princess Grace even went to Rome to implore the pope, but permission has never been granted.

Casiraghi, whose soft, angelic features gave him the appearance of an altar boy, first met Caroline at a fashionable nightclub in Monaco called Jimmy's, after her divorce from Junot. Even though he was a commoner, Casiraghi traveled in the same circles as Monte Carlo's royal household, skiing in St. Moritz and vacationing at plush Riviera resorts such as St.-Jean-Cap-Ferrat, where his family owned homes. He soon decided to break off his engagement with an Italian socialite, Pinuccia Macheda, and won the approval of Prince Rainier to marry his daughter.

Because the Catholic Church refused to grant her an annulment, Caroline and Stefano were married in a simple civil ceremony in December 1983, beneath a portrait of her deceased mother. Andrea, their first son, was born the following year. A daughter, Charlotte, was born in 1986, and Pierre, their younger son, in 1987.

Friends said that under Casiraghi's moderating influence the haughty, temperamental Caroline became more stable and mature. While the couple continued to indulge a taste for lavish parties and fast cars, Casiraghi sought to pursue a business career with the help of his millionaire industrialist father, Giancarlo, who made his fortune in building heating and air-conditioning systems.

The younger Casiraghi launched a successful real estate venture and later started up export companies that sold sweaters and shoes in the United States. At his death he was chairman of Cogefar France, a subsidiary of an Italian construction company owned by the giant car-making firm Fiat. He also held half interest in a real estate company that owns about 3,000 apartments in Monaco and is erecting a massive $160 million housing complex there.

But the prudence Casiraghi displayed as a businessman was abandoned whenever he climbed into cars or boats. He was a passionate offshore racer who had won 12 of 80 races since he began competing in 1984, culminating in last year's victory in the world championship that was held off Atlantic City, N.J.

"He was no hothead, but he loved to take risks," said Franco Bartolino, an Italian journalist who was a close friend. "Stefano told me recently that he intended to quit racing just after this year's championship. He was going too fast at the time of the accident, and I think it was because he wanted to win one last time, at home in Monaco."

Casiraghi and Innocenti had been favored to win the three-stage race, which features sleek, powerful boats that can surpass 130 mph in calm seas. Today's races were called off following the accident, but organizers said the competition would resume Thursday or Friday.