There is a lottery within the boats that take the migrant route across the Mediterranean.

Some get over without incident; some get into distress and are rescued; some sink without witness.

There are a lot of bodies at the bottom of that sea.

And now the list of deaths grows, and probably by a ghastly number after a fishing vessel capsized with what a charity feared was up to 750 on board.

There are already many questions about this incident. Why did the boat start to turn so sharply, apparently causing its own downfall? Who was turning down the offers of assistance, and why? Was the rescue operation adequate – and if not, why not?

But other things are more clear-cut – an overloaded boat, transporting people from a Libyan port to Europe in dreadful conditions.

I know about that, because I’ve seen it up close.

A few weeks ago, cameraman Marc Hofer and I were on board the rescue ship Geo Barents, run by the charity Medecins Sans Frontieres.

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It patrols the Mediterranean, helping ships in distress and, on that sunny Saturday morning, a call for help is exactly what happened.

The Italian coastguard had been alerted to a ship, full of people, that needed assistance. When we got there, it was an extraordinary sight – a fishing boat that had been converted to carry as many people as possible.

It looked almost exactly the same as the one that has sunk off the southern Greek coast – a similar rich blue paintwork, apparently the same structure, with an upper deck and then a hugely crowded main deck.

On the boat we filmed, there was a lower area, called an underbelly, in which hundreds more people were crowded. On both boats, you can see the signs that it had previously been a fishing vessel, and you can also see the rust and wear of a long life.

My guess is that you could probably get about 40 people on to a boat like that before it started to feel too crowded.

At the end of a rescue mission we saw, which lasted several hours, more than 600 people had been transferred off the boat, and on to the Geo Barents.

When we spoke to some of those who were rescued, a story emerged that is now being echoed by survivors in Greece – of a boat that set off from the Libyan port of Tobruk, under the control of a captain.

Hunger, thirst and fear

On the Geo Barents, people told me that they had initially left the port with even more passengers on board but the vessel was so heinously overloaded that it could not be moved safely.

So, at gunpoint, around 150 people were then ordered off before the trip could start again.

They told a story of food and water running out; of people having to sit in the underbelly cramped together and of being ordered to stay in place so the boat would not sway from side to side. And of fearing that they would die on the journey.

We were also told that the captain left the ship one night, collected by a small boat that arrived alongside. After that, the passengers were left to their own devices, even though none of them knew how to control the boat.

Already, similar reports are emerging from the shipwreck – of a captain abandoning his ship, and those on board.

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On board the mission to rescue 600 people
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In both cases, the boats had actually been tracked by Frontex, the European Union’s border agency.

They scan the Mediterranean for vessels such as this and try to monitor their progress, handing on the information to the various nations that run search and rescue operations in the area.

But what is different is that the people we encountered were happy to be rescued. Many thought they were genuinely facing death and they willingly clambered off the boat and into the fast dinghies that took them to safety.

They wanted to reach Italy and, having been rescued by a boat in Italian waters, that was now going to happen.

The boat that sank seems also to have been determined to reach Italy, or perhaps Italian waters.

Maybe that’s why, as it meandered through the Greek zone, it repeatedly denied that it was in trouble – because it wanted to reach Italy’s jurisdiction before accepting any help.

But if the captain had left, then who was making that call? At times like this, there is a surfeit of questions and a paucity of answers.

Whatever – the sight of a dangerously overloaded vessel that won’t ask for help places the authorities in a difficult position.

Maritime law dictates that you must attend to a vessel that says it is in distress, or which is sinking or on fire.

But when a boat denies there’s a problem, then things become more complicated.

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‘A night-time rescue is the worst’

To an extent, all you can do is watch, hope for the best and prepare for the worst. The problem for the rescue teams is that the worst happened at 2am.

The rescue that we filmed happened on a sunny afternoon on a relatively flat sea. But operations at night, especially with so many people in the water, are notoriously difficult.

A person does not have to drift far before they are effectively out of sight. And it is incredible how the break of a wave can hide what’s behind it.

“A night-time rescue is the worst,” one of the crew told me, shaking his head.

This is really a tale of two boats: that look the same, were just as packed as each other and set off from the same place.

On both of them, there were points where the people on board thought they were going to die.

On one of them, everyone survived. On the other, they didn’t. Migration across the Mediterranean really can be a brutal throw of the dice.