Here's what we know about why a luxury sailing boat sank off the coast of Sicily

Emergency workers in southern Italy are still hunting for six people missing after a tornado sank a luxury yacht early Monday - prompting an air and naval operation off the coast of Sicily.

Fifteen people were rescued from the wreckage, according to Italy's Coast Guard. One body was recovered on Monday and four more on Wednesday, after divers struggled to reach deep inside the yacht around 50 meters underwater (approximately 150 feet).

Two Americans and four Britons were among the six initially reported missing - including British tech tycoon Mike Lynch, Jonathan Bloomer, chairman of Morgan Stanley International, and Chris Morvillo, a prominent lawyer.

Here's what we know.

What happened?

A small waterspout - a type of tornado - spun over the Mediterranean island early Monday, likely capsizing the sailing boat amid lashings of rain and strong thunderstorms.

The British-flagged yacht, called the "Bayesian," was anchored about a half a mile from the port of Porticello, on Sicily's northern coast. The vessel sank after its mast broke in half in the storm, Salvatore Cocina, head of Sicily's Civil Protection, told CNN on Tuesday.

Eyewitnesses described furious gales and hurricane-like winds that left an avalanche of debris near the pier.

More than a dozen survivors were spotted in the area hanging onto life rafts, according to the captain of a nearby boat, who steadied his ship to avoid colliding with the Bayesian.

"We got this strong hurricane gust and we had to start the engine to keep the ship in an angled position," Karsten Bower told reporters in Palermo on Monday. "After the storm was over, we noticed that the ship behind us was gone."

Bower and his crew rescued four injured people, he said, before calling Italy's Coast Guard - who later rescued the remaining survivors.

One of those rescued - a child - was airlifted to the children's hospital in Palermo. Eight people were hospitalized in total, according to the mayor's office.

The girl's mother, Charlotte, described how she battled to hold onto Sofia, her 1-year-old daughter, as reported by Italian news agency ANSA.

"In two seconds I lost the baby in the sea, then I immediately hugged her again amidst the fury of the waves. I held her tightly, close to me, while the sea was stormy," she told journalists. "Many were screaming."

The mother and daughter were later reunited with the father, James, according to a doctor at the local children's hospital in Palermo.

"The survivors are very tired and are constantly asking about the missing people," the doctor, Domenico Cipolla, said Monday. "They are talking and crying all the time because they have realized that there is little hope of finding their friends alive."

Italy's fire brigade dispatched helicopters to aid in the search, officials said Monday. After an unsuccessful attempt on Monday, the brigade sent divers to try and enter the sunken ship Tuesday, recommencing the operation on Wednesday.

The depth of the wreck means divers can only work there for limited periods of time, according to Marco Tilotta, an inspector for the diving unit of Palermo's local fire brigade. The Italian fire brigade said Monday its divers had reached the yacht's hull 49 meters (160 feet) below sea level.

The United Kingdom's Marine Accident Investigation Branch (MAIB) said it has deployed a team of four inspectors to Palermo which is conducting a preliminary assessment of the scene.

Who was on board?

Twenty-two people were on board the Bayesian, which was flying under a British flag and had mostly British passengers and crew, in addition to two Anglo-French, one Irish and one Sri Lankan person, a spokesperson for Italy's Coast Guard told CNN.

A troupe of high-profile guests are among those missing, including Lynch, the 59-year-old British tech investor who fought a fraud case earlier this year in the United States - which spiraled from the disastrous $11 billion sale of his company to tech firm Hewlett-Packard (HP) in 2011.

His 18-year-old daughter was also named missing. Lynch's wife, Angela Bacares, survived the accident. Bacares told Italian daily newspaper La Repubblica that she woke up at 4 a.m. local time, as the boat tilted. She and her husband were not initially concerned, she said, but became worried when the windows of the yacht shattered and chaos erupted.

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Bacares spoke to the newspaper while sitting in a wheelchair, at a hospital in the Sicilian town of Termini Imerese. She had abrasions on her feet and bandages on other parts of her body, it reported.

Bloomer, the finance tycoon, and Morvillo, a prominent lawyer - and both their wives - are also among the missing, according to Salvatore Cocina, head of Sicily's Civil Protection.

Morvillo, an American partner at Clifford Chance, was involved in successfully defeating the US fraud case against Lynch in June. Another employee of the firm, Ayla Ronald, and her partner, survived the incident, according to a spokesperson for Clifford Chance.

The body that was recovered from the vessel on Monday was identified as the onboard chef Ricardo Thomas, an Antiguan citizen, Reuters reported.

What do we know about the boat?

Built in 2008, the 56-meter (184-foot) yacht was manufactured by Italian company Perini Navi, Reuters reported. According to the Associated Press, the boat has been available for charter for $215,000 (195,000) per week.

Lynch's wife is linked to the yacht. The Bayesian is held by the company Revtom Limited, according to records from the maritime information service Equasis. The company's latest annual return from April lists Bacares as the proprietor.

"Bayesian," the name given to the vessel, is linked to the statistical theory on which Lynch built his fortune, according to Reuters.

The yacht's mast stood 72.27 meters (237 feet) high above the designated water line, just short of the world's tallest mast which is 75.2 meters, according to Guinness World Records. It was the tallest aluminium mast in the world, the Perini Navi website said.

Perini Navi is known for making "good quality boats," according to Caroline White, deputy editor of BOAT International, a media group serving the superyacht industry.

White told CNN that if the Bayesian dismasted, "it should theoretically become more stable." "But it might be a different story if you are in the middle of a violent storm with incredibly strong winds pinning you to the water," she added.

Dangerous weather conditions

Strong storms across Sicily brought torrential rainfall late Sunday. Initial reports suggest a small waterspout, which developed over the area Monday morning, could have been behind the yacht's sinking.

Waterspouts - one of several types of tornadoes - are spinning columns of air that form over water, or move from land out to water. They are often accompanied by high winds, high seas, hail and dangerous lightning. While they are most common over tropical oceans, they can form almost anywhere.

Matthew Schanck, chair of the Maritime Search and Rescue Council, told CNN that while waterspouts in Sicily are rare, "there is a risk" they can happen - just not every day.

Waterspouts rely on warm waters to gain energy and the Mediterranean Sea has been very hot, reaching a record daily median of 28.9 degrees Celsius (84 Fahrenheit) last week, according to preliminary data from researchers at the Institute of Marine Sciences in Spain.

Local temperatures have been even higher, with waters around Sicily reaching almost 30 degrees Celsius (86 Fahrenheit), almost 3 degrees more than normal, Italian climatologist Luca Mercalli told CNN.

"Warmer oceans have more energy and more humidity to transfer to the atmosphere, the most important fuels for storms," he said.

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