The story of a boat that sank on Lake Maggiore on 28 May has elements of a spy novel.

One was a former agent from Israel’s spy agency Mossad, two were Italian intelligence officers and the fourth victim was a Russian woman.

They were among 23 people aboard the small boat that set out to cruise the lake.

It is no doubt a traumatic event for the survivors and for the families who lost loved ones – but a number of facts about the case have prompted speculation about what exactly happened.

Lake Maggiore and its shoreline are divided between the Italian regions of Lombardy and Piedmont, and the Swiss canton of Ticino.

Several companies that produce technology capable of both military and civilian use are located in Lombardy – and Switzerland is considered a transit country for many intelligence members.

And some of the people on the boat, of both nationalities, had apartments and houses there.

While some reports say the boat ride was an innocent pleasure trip, several major Italian news outlets are reporting that it was a secret work meeting between Italian and Israeli agents.

There is huge interest in the accident and public prosecutor Carlo Nocerino has been tasked with finding out what happened. He says 13 of those on board were Italian agents and eight were Israeli agents. Only the captain, Claudio Carminati, and his Russian wife did not work in intelligence.

The cruise was planned to celebrate the birthday of someone on board. The 15m boat was an apt choice – it was called “Good…uria”, a play on words that means “pleasure”.

But this boat was suddenly hit by a fierce storm with gusts of over 70km (43m) per hour.

“In a matter of 30 seconds, an apocalypse descended upon us,” was how Mr Carminati described the incident, according to the newspaper Corriere della Sera. “The boat immediately capsized, and we fell into the water.”

Mr Carminati told investigators that no bad weather had been forecast. The boat was still on the lake after the scheduled return time.

He is now being investigated for the possible crimes of causing a shipwreck and culpable homicide.

His wife, Anna Bozhkova, was a 50-year-old Russian woman who had a residence permit to live in Italy.

The other victims were Tiziana Barnobi, 53, and Claudio Alonzi, 62 – both members of the Italian secret service.

An Israeli man, 50-year-old Simoni Erez, also died. He was a retired Mossad agent. His name was published in the Italian media, even as media outlets in Israel continued to refrain from reporting it.

The others who were aboard the boat managed to swim to shore or were pulled from the water by other vessels that were dispatched to help or happened to be in the area.

The cause of all deaths is drowning, although Italian news outlets report that no post-mortem examinations have been carried out.

One other aspect to the mystery is that, according to Italian media, the survivors of the disaster seemingly rushed to leave immediately afterwards.

The Italian press reports that the survivors quickly collected their belongings from their hotel rooms and from the hospital where they had received treatment and left.

There is no paperwork about those who received medical treatment.

The Israelis abandoned their rental cars and on Monday they were flown back home on an Israeli plane that picked them up in Milan.

Their identities have not been disclosed, but Mr Nocerino, the public prosecutor, told the BBC it was normal to release only the names of victims and not the survivors.

Israeli Prime Minister Benjamin Netanyahu’s office confirmed on Wednesday that the Israeli man who died was a retired member of Mossad.

“The Mossad lost a dear friend, a devoted and professional member who dedicated his life to the security of the State of Israel for decades,” the office said in a statement.

“Given his service with the agency it is not possible to elaborate on his identity,” it added.

The vessel involved in the incident has not yet been taken ashore. This means the investigation cannot properly begin, Mr Nocerino told the BBC.

“At the moment, it’s entangled at the bottom of the lake and it might take two or three days to bring it up.”

Mr Nocerino added that the boat only had capacity for 15 passengers, but there were eight more than that aboard.

It is being reported that the overcrowding made it harder for the captain to manoeuvre the boat in adverse weather conditions.

Magistrates are investigating why there were more people than permitted aboard – and whether it should have been sailing on the lake at all given the bad weather.

Mr Nocerino said the investigation would examine compliance with security measures, maintenance, certification and insurance of the vessel.

Of course, with so much of the story remaining uncertain, speculation is likely to continue.

The Carabinieri, Italy’s military police, are assisting the investigation. They told the BBC it would examine the boat and the weather conditions – but not what the people were doing on board.