This is the story of the bravest man I have ever met.

I’ve covered many wars and seen many acts of courage. But for sheer grit and determination I’ve never known anyone to compare with Capt Mbaye Diagne, a United Nations peacekeeper in Rwanda.

I was there in 1994, when 800,000 people were killed in 100 days, and I returned to reconstruct the story of this remarkable, charismatic officer from the west African state of Senegal.

The country plunged into war and genocide on 6 April 1994, when the plane carrying the Rwandan president, a member of the majority Hutu population, was shot down. Everyone on board was killed. Within hours Hutu extremists seized power and a tidal wave of murder was unleashed against the minority Tutsi population, and anyone prepared to defend them.

The army came for Prime Minister Agathe Uwilingiyimana that first night.

As gunfire rang out, her five children, the youngest just three, were bundled through a chain link fence to be hidden in a neighbour’s house.

The children were cowering in the brick-built bungalow, occasionally peeping out of the window, when they spotted soldiers looking for their parents.

“There was more gunfire," says Marie-Christine, the prime minister's daughter, who was 15 at the time.

Agathe Uwilingiyimana was a moderate Hutu, not a Tutsi, but she was killed because she was ready to share power with them. Had the killers found the children they would have been slaughtered too.

Hours later, when UN soldiers arrived to pick up UN aid workers from the compound behind the prime minister’s residence, they discovered Marie-Christine and her brothers still hiding in the bungalow.

A fierce argument broke out about what to do with the children. It was not clear that the UN soldiers were authorised to move them, says Adama Daff, one of the aid workers, but “on humanitarian grounds we definitely could not leave them there”.

It was extremely dangerous to travel anywhere. Roadblocks manned by Hutu killers had already appeared, and the armoured personnel carriers which were supposed to have taken UN aid workers to safety had not shown up.

In the end, Daff says, it was decided that Capt Mbaye, an unarmed military observer, would take the children in his unarmoured car to the relative safety of the nearby UN-guarded Hotel des Mille Collines.

“He decided to load the kids up,” says Gen Romeo Dallaire, the Canadian commander of the small and poorly equipped UN force. “He hid them under a tarpaulin and just drove like stink.”

They were the first of many people Mbaye took to the Hotel des Mille Collines - an unremarkable edifice of glass and concrete set on a hill overlooking the capital Kigali, but one of the few sanctuaries for Tutsis in the city.

Capt Mbaye Diagne was in his mid-30s, from a small village in northern Senegal, and a man of immense charm. Tall, gap-toothed and easygoing in Aviator sunglasses, his humour put people at their ease even in one of the darkest chapters of modern history.

The first, bloody days of the genocide felt like pandemonium.

There was hot lead flying in all directions and bodies lying, sometimes piled up, on the sides of the roads.

The terrifying roadblocks were mainly manned by the Hutu Interahamwe militia. The word means “those who work together”- and the work was killing Tutsis with machetes, knives and sticks. I saw one man attack another in the head with a screwdriver.

Radio stations urged them on, calling for the death of Tutsi “cockroaches”.

The shooting down of the president’s plane had rekindled a civil war between the government army and rebel forces of the Rwandan Patriotic Front (RPF) which had been briefly on hold following a tentative peace deal. Led by the Tutsi Paul Kagame, the RPF was advancing on the capital, saying it would stop the massacre.

In between the two sides was the beleaguered UN force. Its vehicles were sometimes attacked by Hutus - especially if the militia thought there were Tutsis inside them.

Within the first 48 hours, a lot of the unarmed military observers like Mbaye - especially those outside the capital - disappeared. “It took us nearly a month to find some who had gone to different countries,” says Dallaire. “Some ended up in Nairobi before we knew where they were.”

With virtually no-one to defend them, tens of thousands of Tutsis sought refuge in churches, but even here they were not safe. One of them, Concilie Mukamwezi, went with her husband and children to the Sainte Famille church, a large religious compound in the centre of Kigali. She remembers her time there with digital clarity.

“I had just bought some laundry soap from a stall when a priest in military uniform came up to me,” she says.

“He had four militiamen with him and he was armed with a Kalashnikov rifle, a pistol and grenades.

“This priest accused me of being a collaborator with the rebels.

Incredible though it may seem, some Hutu clergy were collaborating in the genocide, and some were even taking part.

One of Mbaye’s jobs was to be the eyes and ears of the UN mission, and he made it his business to check occasionally on the people sheltering at Sainte Famille.

He knew Concilie by sight because before the genocide she had worked at the office of the national telephone company, Rwandatel, where he paid his phone bills. And by coincidence he happened to walk into the church compound at her moment of need.

“Captain Mbaye ran over and stood right between the priest and I,” says Concilie. “He shouted, ‘Why are you killing this woman? You must not do this because if you do the whole world will know.’” The priest backed down.

There was no large-scale killing inside the Sainte Famille compound, partly as a result of the efforts of Mbaye and the other UN peacekeepers - although plenty took place just outside.

In many churches where people had taken sanctuary, soldiers and militiamen broke in and massacred them in the pews.

Other desperate Rwandans attempted to take advantage of rescue operations launched for the country’s expat community.

Ancilla Mukangira, a Rwandan working for a German aid agency, made her way to the American Club in the mistaken belief that the Americans would give her a place in one of the vehicles due to leave the country.

“I went in to register for the convoy,” she tells me outside the old club, which is today a Chinese restaurant. “But they said no Rwandans were allowed, and told me to leave.”

Ancilla was standing, crying, on the pavement outside, when Mbaye approached her.

“What are you doing here?” he asked. “If they see you they will kill you.”

She told him she had been kicked out. He was appalled, and could barely believe it, she says, but then offered to help her himself.

“Mbaye was shocked by the behaviour of the Wazungu [whites],” says Andre Guichaoua, a French academic staying at the Mille Collines hotel, who got to know Mbaye well in the first few days of the genocide.

French, Belgian and Italian troops were flying into Kigali - but only to save their own nationals.

“Because if you had put the French and Belgian soldiers alongside the United Nations troops it would have been perfectly possible to confront the army and militia who were directly involved in the massacres," Guichaoua says.

“There was no co-ordination - and Mbaye was deeply horrified by this.”

In fact, there was very little co-ordination even within the UN system. While officers like Mbaye were bravely protecting those they could, UN bosses in New York were still arguing how - or even if - to support them. Soon after hostilities began they actually reduced the number of UN troops on the ground from 2,500 to less than 300.

The US, meanwhile, was determined to avoid putting boots on the ground. It was just six months after the humiliation of its forces in Somalia when 18 US rangers were killed in an incident which became known as Black Hawk Down.

So Mbaye drove Ancilla Mukangira to the Hotel des Mille Collines, past the militia men who were waiting at the gate to kill the Tutsis inside.

He told her to stay in his room and not open the door to anyone, returning only late at night, with an extra mattress for her to use.

“He saw me reading my Bible,” Ancilla remembers.

I had got to know Mbaye a little myself. Soldiers are normally wary of journalists, but, in this, as in other ways, he was different.

One day, we drove together in his white UN car to gather information about an orphanage in a suburb of the city called Nyamirambo, where it was believed several hundred vulnerable children might be hiding.

On our way there, we were stopped at a militia roadblock. One of the militiamen walked over to the car and leaned through the window holding a Chinese stick grenade. It looked like an old-fashioned sink plunger, but instead of having a rubber sucker on the end of a stout stick, it had a bomb.

He waved it at me.

“Who’s this Belgian?” he asked menacingly.

The militia considered Belgians, the former colonial power in Rwanda, to be their enemy. They had recently killed 10 Belgian soldiers, who were part of the UN force, calculating that this would make the entire Belgian UN contingent leave Rwanda – which it did.

I was terrified I was about to be killed, but Mbaye looked at the man, smiled, and cracked a joke.

“I’m the only Belgian in this car. See?” he said, pinching some of the jet-black Senegalese skin on his arm. “Black Belgian!”

The joke broke the tension of the moment. Mbaye then ordered him out of the way, the militiaman instinctively obeyed - and we drove on.

“He loved joking with people, he loved talking,” says one of his former comrades in the UN mission, Babacar Faye, now a colonel in the Senegalese army.

Mbaye was a devout Muslim, but he carried alcohol in his UN 4x4 to buy the lives of people he was taking through the deadly checkpoints.

“In his car, he would often have cases of beer, bottles of whisky and lots of packets of cigarettes,” says Faye. “And he always had wads of cash.”

I once saw a list of names on a scrap of paper that had fallen out of his pocket. It was a list of first names –“Pierre”, “Marie’ - with sums of money written next to them - $10, $30 and so on.

These were his records – the amounts he had paid, often on someone else’s behalf, to get people through the checkpoints.

He sometimes even gave away his military food rations – and when his colleagues found out, they donated theirs to add to the valuable stash on the back seat of his car.

“When he was stopped at these roadblocks, the militiamen would say ‘Boss, I’m hungry’ or ‘Boss I’m thirsty’ so he’d give them a cigarette, or if it was one of the militia chiefs he’d give a beer or a whisky,” says Faye.

“This allowed him to go everywhere without making the militiamen too angry. And that’s how he saved people the militia wanted to kill – five or six people in his car at a time.”

As time went on, the war split Kigali into two zones – one controlled by the government, the other by the RPF.

The Hotel des Mille Collines was in the government-controlled zone, right next to a barracks where some of the militia leaders were based. But thanks to its armed UN guards, many Tutsis and moderate Hutus did what they could to get inside. Most had to have money or contacts.

The prime minister’s children were smuggled out of the hotel after a few days – hidden under suitcases in the back of a UN vehicle. They were taken to the airport and flown to safety, still dressed in the pyjamas they were wearing when they fled their home.

But more and more people arrived at the hotel and conditions steadily worsened. Water supplies were cut off, forcing those sheltering there to drink water from the swimming pool. At first they would boil it, but after the power was cut too, they couldn’t even do that.

On one occasion Mbaye and other UN officers tried to organise a convoy of UN trucks from the Mille Collines to the airport. A doctor, Odette Nyiramilimo was on one of the lorries with her family, while Mbaye was in the lead vehicle.

The convoy made it out of the hotel gates, but it only got a few hundred metres down the road before it was stopped by a crowd of militiamen.

A government propaganda radio station had got hold of the list of the people in the lorries, and was reading it out on air, whipping the militia into a frenzy.

“They were trying to pull us off the lorries,” recalls Dr Nyiramilimo, “shouting ‘Kill the cockroaches!’

“Then Captain Mbaye ran up. And he stood between the lorry and the militiamen holding his arms out wide.

Eventually, Mbaye, along with other Senegalese officers, dissuaded the militia from killing the people on the convoy. But the crowd of militiamen was too big to drive through so they had to turn the convoy back to the hotel. They had not been able to get to the airport and out of the country, but they were alive.

Back at the Mille Collines, while the doctor was giving first aid to passengers who had been dragged from the vehicles and attacked, Mbaye came up to her.

“He seemed shocked,” Dr Nyiramilimo says. “He was saying, ‘They almost killed you, you know, they really wanted to do it.’ And he was upset – he was almost crying.

Dr Nyiramilimo and Ancilla Mukangira eventually left the hotel in later convoys. The UN organised “swaps”, with Tutsis trapped on one side of the front line exchanged for Hutus stranded on the other. In this way thousands were saved.

We will never know exactly how many people owe their lives to Mbaye.

His old friend Col Faye puts it at “400 or 500, minimum”. He believes all of the people in the Hotel des Mille Collines would have been killed had it not been for Mbaye’s pivotal role in defending it.

An official estimate by the State Department in Washington, which in 2011 honoured Mbaye with a Tribute To Persons Of Courage certificate, says the figure is “as many as 600”.

But the American Fulbright Scholar Richard Siegler, who lives in Rwanda and plans to publish a book on Mbaye, thinks the correct figure may be 1,000 or more.

“The full extent of Captain Mbaye's actions has yet to be recognised, because those who saw him act only saw a small part of what he was doing,” Siegler says.

It would be wrong to suggest that Mbaye was the only one to have saved lives in Rwanda in 1994 - there were countless cases of extreme bravery by Rwandans themselves.

But in all of the years since the genocide, researchers have pored over the details of what happened, and none has found anyone involved in as many rescues as Capt Mbaye Diagne.

His luck finally ran out on the morning of 31 May 1994.

By this time the RPF had the upper hand but government forces were making a last stand in central Kigali. Almost every day there were big battles in the city – fights so intense that the sounds of individual guns firing merged together to make a deafening noise like rolling thunder.

It was on one of these days that Mbaye was asked to take an important written message from the head of the government army, Augustin Bizimungu, to the UN commander, Romeo Dallaire, who was based in the zone now held by the RPF.

Mbaye would have to leave the government-controlled sector by driving through a government army checkpoint.

He stopped at the checkpoint and a mortar round exploded on the road a short distance from his car.

Shrapnel tore through the bodywork.

Mbaye was hit and died instantly.

“It was a very, very difficult day,” says Dallaire, who is now a senator in the Canadian Parliament. “[There were] so many, but it stood out because we lost one of those shining lights, one of those beacon-type guys who influences others.”

Mbaye was part of a small group who had been willing to risk their lives to save others, says Dallaire.

“He had a sense of humanity that went well beyond orders, well beyond any mandate.

And he had been about to go home.

“There are only 12 days left before my part in this mission ends,” he had told his wife, Yacine, on the phone three days before he was killed. “Then I will be back in Senegal. So you must pray for us.”

In that last call home to Dakar, he talked a lot about death. “That really upset me,” says Yacine. “He never used to talk like that before. I think the things he saw over there deeply affected him.”

Their two children, a boy, Cheikh and girl, Coumba, were just two and four years old when their father died. It would be two years before Yacine could bring herself to tell them the truth. “Daddy will be home when his mission ends,” she would tell them.

I asked Yacine how she had held the tragedy inside her and not shared it with her children.

“Yes, it was hard, but they would not have understood,” she says. "It was the right thing to do – to protect them from it until they could understand.”

The daughter of the assassinated prime minister, Marie-Christine Umuhoza, is now married with two children of her own.

She and her brothers were flown to France, but the country which had provided a home for the wife and family of the murdered president rejected the children of the murdered prime minister. Instead they ended up as refugees in Switzerland.

Marie-Christine lives in Lausanne, where she works as a psychiatric nurse. She had never spoken publicly about the events of 1994 before, but she told me her chilling tale with great poise and dignity.

She seems to have been able to put a tragic part of her life to one side and move on.

“When I agreed to speak to you, I did it in part so I could pay tribute to the memory of Captain Mbaye,” she says.

I heard about Mbaye’s death after noticing an unusual amount of chatter on the UN walkie-talkie network. I heard soldiers talking about a serious incident at a government roadblock in which a UN military observer may have been killed.

“Oh God, I hope it’s not Mbaye,” said a UN aid worker. But he was in denial – he knew it was Mbaye.

I rushed to the roadblock with a Canadian UN officer who also knew but couldn’t bring himself to say it.

When I found the car the body had been taken out. There was blood on the seat and in the footwell.

The next day, when his body was being taken to a plane at Kigali airport for repatriation to Senegal there was no coffin available – the UN mission was operating on such a shoestring, and had been so abandoned by the rest of the world, that Mbaye was wrapped in a large piece of the blue plastic sheeting the UN normally uses for sheltering refugees.

A UN flag was placed on top.

Just before the body was loaded, one of the other Senegalese military observers, Capt Samba Tall, approached me.

“I am a soldier,” Capt Tall said, “but you are a journalist. You must tell the story of Capt Mbaye Diagne.”

Then Capt Tall and I both broke down in tears.