More than 180 feet long, with a mast towering about 240 feet and a keel that could be lowered for greater stability, the Bayesian luxury yacht did not, in the eyes of its maker, have the vulnerabilities of a ship that would easily sink.

“It drives me insane,” Giovanni Costantino, the chief executive of the Italian Sea Group, which in 2022 bought the company that made the ship, said after its wreck last week. “Following all the proper procedures, that boat is unsinkable.”

But the $40 million sailing yacht sank within minutes and with fatal results: seven dead, including the British technology billionaire Michael Lynch, his teenage daughter, four of Mr. Lynch’s friends and a member of the crew. Fifteen people, including the captain, escaped on a lifeboat.

Mr. Lynch had invited family, friends and part of his legal team on a cruise in the Mediterranean to celebrate his acquittal in June of fraud charges tied to the sale of his company to the tech giant Hewlett-Packard.

The Italian authorities have opened a manslaughter investigation, searching for answers from the survivors, the manufacturer and the wreck itself. They face a range of questions and possible factors.

An ‘earthquake’ in the sky?

When the Bayesian sank around 4 a.m. on Aug. 19, the waters in its area, about half a mile off the Sicilian port of Porticello, were transformed by an extremely sudden and violent storm, according to fishermen, a captain in the area and meteorologists.

But what kind of storm is still a mystery, compounded by the fact that a sailing schooner anchored nearby did not have its own disaster. Also unclear is whether the crew was aware that the Italian authorities had issued general warnings about bad weather the night before.

Karsten Börner, the captain of the nearby passenger ship, said he’d had to steady his ship during “really violent” winds. During the storm, he said, the Bayesian seemed to disappear behind his ship.

Severe lightning and strong gusts were registered by the Italian Air Force’s Center for Aerospace Meteorology and Climatology, according to Attilio Di Diodato, its director. “It was very intense and brief in duration,” he said.

The yacht, he said, had most likely been hit by a fierce downburst — a blast of powerful wind surging down during a thunderstorm — or by a waterspout, a phenomenon similar to a tornado over water. His agency put out rough-sea warnings the previous evening, alerting sailors about storms and strong winds.

Locals have said the winds “felt like an earthquake,” and a fisherman in Porticello told an Italian newspaper he had seen a waterspout shortly before 4 a.m., and a flare go off around 4:10. He ventured to the site once the weather had calmed about 20 minutes later, he said, finding only cushions and planks.

Waterspouts are relatively common in the Mediterranean, meteorologists say, but like tornadoes their location and intensity are hard to predict.

The Italian authorities, however, have said they believed a downburst was more likely. They have so far declined to say whether investigators had seen any structural damage to the hull or other parts of the ship.

Open hatches or doors?

The boat executive, Mr. Costantino, has argued that the Bayesian was an extremely safe vessel that could list even to 75 degrees without capsizing. His company, the Italian Sea Group, in 2022 bought the yacht’s manufacturer, Perini Navi, which launched the ship in 2008.

Mr. Costantino said that if some of the hatches on the side and in the stern, or some of the deck doors, had been open, the boat could have taken on water and sunk. Standard procedure in such storms, he said, would be to switch on the engine, lift the anchor and turn the boat into the wind, lowering the keel for extra stability, closing doors and gathering the guests in the main hall inside the deck.

At a news conference on Saturday, almost a week after the sinking, investigators said the yacht had sunk at an angle, with its stern — where the heavy engine was — having gone down first. The wreck was found lying on its right side at the bottom of a bay, about 165 feet deep.

Water pouring into open hatches or doors could have contributed to the sinking, experts say, but that on its own may not account for the speed at which such a large boat vanished underwater.

Asked about the hatches at the news conference, the authorities declined to comment on whether they had been found open at the wreck.

The authorities have also not specified whether the boat had been anchored, whether it was under power at the time or whether its sails had been unfurled.

A retracted keel?

The Bayesian had a keel — the fin-like structure beneath a boat that can help stabilize it — that could be retracted or extended, according to its manufacturer. On some yachts, keels can be raised to let the large vessel dock in shallower water, and extended downward to help keep a boat level.

But like the hatches, the status of the keel alone may not explain why a large ship sank with such precipitous speed. Investigators have not disclosed what divers may have seen at the wreck, aside from saying divers had faced obstacles like furnishings and electrical wiring in tight quarters. Officials want to raise the wreck to better examine it, a process that may take weeks.

Human error?

Ambrogio Cartosio, the prosecutor in charge of the case, said at the news conference that it was “plausible” crimes had been committed, but that investigators had not zeroed in on any potential suspects.

“There could be responsibilities of the captain only,” he said. “There could be responsibilities of the whole crew. There could be responsibilities of the boat makers. Or there could be responsibilities of those who were in charge of surveilling the boat.”

It remains unclear what kind of emergency training or preparation took place before the disaster, or what kind of coordination there was during it. So far, none of the surviving crew members have made a public statement about what happened the night the ship sank.

Prosecutors said they want to ask more questions of the captain and crew, who have been in a Sicilian hotel with other survivors. They said that neither alcohol nor drug tests had been performed on crew members, and that they have been allowed to leave Italy.

Prosecutors also said they were also investigating why the captain, an experienced sailor, left the sinking boat while some passengers were still on board.

Besides possible manslaughter charges, the authorities are investigating the possibility of a negligently caused shipwreck.

The bodies of five passengers were found in one cabin, on the left side of the yacht, the authorities said. The five were most likely trying to flee to the higher side of the boat and were probably sleeping when the boat started to sink, they said.