Image copyright Reuters Image caption Muaz Ayimo, from Ethiopia, is one of 37 survivors of the shipwreck

An investigation by Reuters and BBC Newsnight has, for the first time, revealed the untold story of what is understood to be the biggest single shipwreck of a migrant boat in 2016. More than 500 people are believed to have died, but there has been no inquiry into the deaths.

On 9 April this year, in the hold of a trawler, more than 300 people sat in the dark, feeling the boat rock in the rough sea.

Among them was Muaz Ayimo, a young Ethiopian, holding on to his baby girl and his wife.

On deck, a "feeder" fishing boat carrying 200 more migrants, chiefly from the Horn of Africa, was roped to the trawler and the fresh cargo was taken on board.

The 70ft trawler, now horribly overloaded, listed to one side then the other as the panicking migrants tried to maintain the vessel's balance.

But 500 people weigh roughly 10 tonnes and that weight, shifting fast, is a killer at sea.

Suddenly, there was a terrible cracking sound of broken timber and then the trawler capsized.

In panic, as the trawler turned, people fell on top of Muaz, and his daughter and his wife slipped out of his hands: "I keep hearing their voices and think about the times we had together."

Muaz, a strong swimmer, managed to reach the surface where he swam through dead bodies to reach the people smugglers' fishing boat.

There was no moon but the fishing boat had one light which he swam towards. He was hauled in and, in turn, he helped two of his friends to board.

He tried to help a third but then a smuggler hit him, then threatened him with a knife - and so the man fell back into the sea.

The fishing boat sailed off, leaving around 100 people still alive - still swimming - to drown.

Thirty-seven people survived the sinking - including Muaz. His wife and daughter did not. They and more than 500 others perished at sea.

Image caption Muaz's wife and daughter both died in the sinking

The trawler was wickedly overloaded: the charge for that is manslaughter. The abandoning of the 100 people swimming in the sea: murder. A charge that a lawyer acting on behalf of some of the families of the Egyptian victims hopes to pursue in court.

But no such charges of manslaughter or murder have been brought, because there has been no proper investigation.

Greece, the country where the 37 survivors landed, has not investigated - nor has any United Nations body, the European Union's frontier agency, the EU police agency, Europol, nor any maritime agency, Nato, nor the two EU naval task forces in the Mediterranean.

The UN estimates that 4,663 people have died this year attempting to cross the Mediterranean. That makes it the most deadly year on record.

And - just as with this sinking - the disasters are routinely not investigated. They fall into a gap in international law.

Origin of those who died in the sinking:

190 Somalis

150 Ethiopians

170 Egyptians and other nationalities

The UN Convention on the Law of the Sea places no requirement on either individual countries, or international bodies, to investigate - though the Palermo Convention of 2000 does require nations to do all they can to fight illegal migration.

Back in April, the UNHCR did report that the trawler had come from Libya, a broken state - the effect being that any investigation would be without purpose.

But they had unwittingly swallowed an untrue account told by survivors, a lie drummed into them by the smugglers. The trawler and its human cargo had all left from Egypt, a police state where very little moves without the knowledge of the authorities.

The consequence of the non-investigation for the relatives of the dead is grief without end.

Media playback is unsupported on your device Media caption Some extracts from John Sweeney's report on the "forgotten shipwreck"

Newsnight's John Sweeney and Stephen Grey from Reuters will be taking your questions on their investigation on the BBC Newsnight Facebook page at 13:00 GMT on Wednesday 7 December

Iman Nasr Taha, fought back tears as she listed some of the nine boys who had left her village in the Nile Delta for Italy, including her own son, Hamed Mohamed Hamed, 17 years old.

She said: "There is no inquiry because the Egyptian government does not care. Nobody told us anything about what happened. Are they alive? Are they dead?"

In this investigation, BBC Newsnight and Reuters have pieced together the details of what happened that dark night eight months ago - including information on the smugglers, the ship that sank, and the brokers.

Our investigation has established the following: that the trawler - called, we believe, the Abu Nawal - left the Egyptian port of Rashid, east of Alexandria.

Migrants from Ethiopia, Somalia, Egypt, Eritrea and Sudan paid around $2,000 (£1,600) a head to brokers in Cairo to be shipped to Italy, creating a gross income for the people smugglers of around $1m (£785,000).

Image caption Migrants gathered in Alexandria before setting off from a nearby port

The migrants were then bussed to Alexandria and then, at night, taken off Miami Beach, Alexandria, in small dinghies each carrying around 30 people to an in-shore fishing boat. That ferried the migrants to the trawler, lying off the Egyptian coat in international waters.

At around 02:00 on 9 April, the fishing boat made a rendezvous with the trawler. When the 200 new migrants scrambled on to the trawler to join the 300 already on board, it started to list towards the fishing boat.

To correct that, the crew yelled "balance, balance" and when the migrants shifted to the other side, too fast, in too great a mass, it capsized.

The crew told the 37 survivors to tell the authorities in Greece that they had come from Libya. They were warned that if they told the truth, they would be returned to Egypt - so the survivors, too, share some responsibility for allowing the perpetrators of the tragedy to go unpunished.

One grieving father, Abdul Hamid, a civil servant in Alexandria, set out to discover who was responsible for his son Ibrahim's disappearance.

Image copyright Marina Militare Image caption The ship capsized in a similar way to this sinking of a migrant boat

He identified some of the smugglers and took his evidence to the police.

Curiously, in July, an Alexandria court convicted seven people in absentia not of manslaughter or murder but fraud.

The convicted - still at large - included alleged smuggling leaders Ismail al Bougy, 41, and"Dr" Ahmed Obeid, 51. According to Egyptian police, they have both been sentenced in absentia several times by Egyptian courts for offences connected to illegal migration.

Newsnight paid a visit to Bougy's house in a gated community in Alexandria. No one was in. Obeid's house was seen as too dangerous to approach, and he did not respond to messages left at his home.

The Egyptian ministry of justice said in a statement that a law was ratified in November of this year to help combat illegal migration. They could not say what inquiries had taken place into the 9 April sinking but added: "If the occurrence of such a crime is proven, Egypt certainly will not hesitate to conduct the necessary investigations to uncover it."

Image caption Ibrahim died in the sinking and his father wants to see justice done

The UNHCR which, in April, had put out an incorrect statement asserting that the trawler had left from Libya told us: "The UNHCR is not a law enforcement agency and investigating sea disasters or transnational organised crime is beyond our means, mandate and expertise.

"But with many thousands of lives having been lost on the Mediterranean… the need to bring to account those involved in the organised trafficking and smuggling of people couldn't be clearer. The loss of life at sea has been truly appalling and we have been very vocal in denouncing it."

But Rob Wainwright, head of Europol, said: "The absence of any clear answers in this case is uncomfortable."

He welcomed the Reuters-Newsnight investigation and said that he would like to receive our evidence and promised "to look at it again".

Back in Cairo, we set out to challenge one of the brokers - a Somali, who had sold places to the migrants on the doomed trawler.

Within seconds of getting out of our vehicle, we were surrounded by secret policemen, some carrying guns, and held.

Stopping journalists from investigating a mass loss of life is more important to the Egyptian authorities, it seems, than hunting down the perpetrators.

John Sweeney and James Clayton were reporting for BBC Newsnight. Watch their full investigation at 22:30 on BBC Two.

Read more: Europol set to investigate mass drowning

John Sweeney and Stephen Grey from Reuters will be taking your questions on their investigation on the BBC Newsnight Facebook page at 13:00 GMT on Wednesday 7 December

Our World: The Forgotten Shipwreck is on the BBC News Channel at 21:30 Sunday 11 December and on BBC World News. It will be available to watch later via BBC iplayer