The man from the Libyan security services held a black beret over his nose. We had been walking in the heat for more than an hour across the dunes. Along the way we had seen discarded T-shirts and jeans, the occasional rucksack, and now this. The body was fresh, around two weeks old. It was that of a young man, clothed in a chocolate-brown T-shirt and faded jeans. There was no visible injury, no blood. He was lying on his back, propped awkwardly against the hot Saharan rocks.

I crouched beside an outstretched arm and saw something had fallen from his grip. It was a plastic water bottle, empty and half submerged in the sand. The guard pointed back up the path. "Many more this way, many more," he said.Our trip to this remote part of Libya took place three weeks before the celebrations for Colonel Gaddafi`s 40 years in power.

While the rest of the world`s press struggled to get entry visas, and news of the pending release of the Lockerbie bomber from a Scottish prison not yet broken, we were embedded with Libya`s security services in the Sahara desert.

The migrant route takes in Ghana, Burkina Faso and Niger

We were not looking for al-Qaeda, even though its followers are known to operate close to here on the Algerian border, we were looking for African migrants. Every year, around 40,000 of them cross the Sahara on their way to Europe. We found ourselves in the middle of the most dangerous migration route in the world.

Most of those who attempt to cross here are economic migrants, but some are political refugees, fleeing conflict and persecution in places like Sudan, Eritrea and Somalia. We had air-conditioned vehicles and huge tanks of water, but the migrants cross this part of the desert on foot. When the British Army patrol in similar conditions in Afghanistan, each soldier is supposed to drink 14 litres of water during a day-long operation. These migrants walk in the desert for four days, sometimes with just a litre or two of water, so that they can keep the weight down.

The next body we came across lay with his wrist across his forehead, like he was wiping away the sweat. He looked about 18. His jeans had been torn around the left calf, probably by a jackal, and the flesh was torn away down to the bone. Nearby, there was another empty water bottle. It is a story that has come to dominate my life for the last two years.

`We walked`

In 2007, an extraordinary photograph in the morning papers set off my work on this story. It was an aerial shot of a tuna net, in the middle of the Mediterranean Sea. Hanging from it were 27 men, clothed but with no sign of any boat. It seemed they had fallen from outer space.

When I finally tracked them down, to a hostel in Italy, it turned out they had been trying to cross the sea to Europe when their boat had capsized. They had clung onto the net for three days without proper food or water. None of them could swim, and they were so unfamiliar with the sea, they believed the tuna in the net were man-eating fish.

"But how did you get the Mediterranean in the first place?" I asked. They looked at each other, puzzled. "We walked across the Sahara," they said. The youngest was a 19-year-old, from Ghana, named Justice Amin. As we talked I realised the journey he`d taken, that they`d all taken in the hope of reaching a "promised land" was of biblical proportions. They had not just risked their lives crossing the sea, they had first risked them on that walk across the desert. All of them had lost friends. They had been reduced to drinking their own urine on the Sahara, and drinking seawater on the Mediterranean. A quarter of those who attempt the journey each year end up dead.

Justice Amin finds life in Naples difficult and warns others not to follow

Justice`s story was particularly compelling. He had been raised by a witchdoctor, who spoke in tongues and sacrificed chickens on a clay idol outside their hut. Yet he could still speak four languages, and quote lengthy tracts from both the Koran and the Bible.

He told me about the journey to Europe - about the nomads who lead men into the desert and then abandon them without water, the people smugglers who trick migrants onto boats without enough petrol to reach the other side and the Libyan border patrols who dress "like ninjas" and capture migrants on the dunes.

I wrote a book about him, called "I am Justice", and now here I was, on the Sahara, with the very patrols he had tried to evade. We were back in the air-conditioned jeep when the radio message came through. They`d just spotted 21 migrants walking across the sand a couple of miles away. If caught they would be thrown into a Libyan prison.

But if they were not stopped, they might just make it to the Mediterranean and the next step on this deadly journey. Or, they might end up as corpses propped against Saharan rocks, arms reaching out for empty bottles.