Terry Lloyd lay dead in the back of a makeshift civilian ambulance, a bullet wound in his shoulder and another, fatal, wound in the back of his head.

As the vehicle sped away towards Basra, it left behind the ITN car in which the 50-year-old journalist and his cameraman Daniel Demoustier had been travelling - now a burned-out wreck.

Nearby was a white pick-up truck, mounted with a machine gun, which had carried seven Iraqi militia. Lloyd's colleagues, translator Hussein Othman and cameraman Fred Nerac, had been captured by the militia on the road to Basra.

Othman lay dead beside the corpses of four of the militia, while Othman and Nerac's ITN vehicle was a little distance away, riddled with bullets. Five hundred yards off, stood a battalion of US tanks.

It was March 22, 2003, days after the American invasion of Iraq had begun, and something horrendous had just taken place.

Daniel Demoustier escaped to tell how the ITN team had been caught in a deadly crossfire between Iraqi fighters and US Marines. But the Americans were admitting nothing and no one was sure exactly what had happened. It had become my job to find out.

What I learned has never been publicly revealed, other than to the Oxford coroner's court and to executives within ITN. Now that the coroner has given his verdict of unlawful killing, however, I feel I can finally tell the full story.

It casts new light on how Terry Lloyd was killed and also confounds the perception of many that it was simply trigger-happy Americans who shot him. My investigation exposes for the first time why the American troops began firing and exactly how Terry was killed. I found out that he had not died instantly, something that I knew would bring great pain to his family back home.

I was in my Kuwait hotel room that day, waiting for a wailing air-raid siren to stop, when a phonecall came from a colleague at another television network. "Is it one of your crews that has been hit in Iraq?'

I had covered wars for 20 years and now, at 39, I was working as a producer for ITN. I felt my stomach tighten, and a quick call to my foreign editor answered the question: Lloyd, Demoustier, Nerac and Othman had been involved in what sounded like a fatal attack. She told me Daniel Demoustier had survived, but she was desperate to find out what had happened to the others.

Grabbing a cab, we raced towards the Iraqi border to fetch Daniel, who had been rescued from the scene by Mail on Sunday journalist Barbara Jones. He was brought across the border by British Royal Military Police to meet us, and he was a gibbering mess, talking 19 to the dozen and obviously traumatised.

He didn't know if Terry or the others were still alive. Then, a few hours later, my London office called. They had just seen a picture of Terry's body in a hospital in Basra. The station had made its own inquiries at the Pentagon and in London, but no one was getting any answers. Indeed the Americans initially denied, in writing, that their troops were even present. ITN wanted me to mount an investigation.

Terry was a veteran reporter whom I knew a little. I had met him first in Kosovo. Just two days before he was killed, I had smuggled some whisky into dry Kuwait and we had shared a furtive night-time drink.

He was his usual ebullient self - determined to do his job independently and scrupulously and to find the most powerful human stories. Learning how he had died was going to be difficult and painful.

For the first two weeks, I worked on the phone from my hotel room, trying to reach members of the Red Crescent ambulance service in Basra. I had been told they might have been at the scene. But there were no phones working in Basra and the British Army was weeks away from entering the city.

Amid the chaos, we managed to collect Terry's body from Red Cross workers who had a base in Kuwait. But we were still no closer to knowing how he died.

I hired Arabic translators to check the Basra hospitals for our missing men or any eyewitnesses.

We also hired extra ex-SAS security advisers to enter Basra and search for the team, while I prepared 'reward' posters with huge photos of Fred and Hussein for them to paste all over the city. But it seemed an impossible mission.

Then we got our first lead. An American journalist who had been embedded with American troops sent us some photographs of the ITN vehicles on the road to Basra on the day of the incident.

The journalist had arrived at the scene just hours after the shooting. The photographs didn't show any injured people or bodies and we wanted to know if he and his crew had seen anything else.

It turned out they had been with the same battalion that had fired on our team. But they had been stopped by US Marines from going near the ITN vehicles. In fact, the sergeant-major had threatened to leave them in the desert if they searched for survivors.

I was tearful and angry and began to be deeply suspicious that the US military was mounting a cover-up. But at last I had somewhere to begin.

I thought of Terry, his bravery and his determination to get to the bottom of every story, and the possibility that his family might never know what had really happened. To find the truth, I would have to put myself in danger.

So, accompanied by bodyguards, I made the 15-hour drive to Baghdad with another camera crew - I was also continuing to report news. It was extraordinary luck that, on my first assignment in Baghdad a couple of days later, I met a spokesman from the US Marines who was part of the same tank battalion said to have been at the scene.

He confirmed that he knew about the incident and directed me to Captain Greg Poland, whose tanks had fired on Terry and the team. It was agreed I would meet him at a US base near the centre of Baghdad.

American troops had just entered the city and the situation was chaotic and becoming increasingly dangerous. We were shot at almost every day. As we drove through the Western suburbs, I saw a man with a rocket-propelled grenade standing only feet from our vehicle.

There was another man with a machine gun. Across the street a burning building was being looted. We were in an armoured car driven by our private security guard and I was frankly scared.

For reasons I will never understand, our driver sped towards the man with the machine gun. He opened fire, emptying his magazine. About 20 bullets hit our vehicle but thank God no one was hurt. I was petrified.

All around us, buildings had been bombed and looted, but this five-storey block - part of a former Iraqi government ministry with a vast concrete car park - had been commandeered by the Americans and was intact.

It was a hot, humid and dusty day and Captain Poland was ill after a bout of food-poisoning. A battle-hardened but handsome man, he was head of Red Platoon, Delta company, First Tank Battalion of the First Marine Division. He wore camouflage fatigues and looked pale and worn. But he was also honest.

He told me his men were having sleepless nights, fearful that they might have killed a journalist. But he also wanted to explain why the tanks had begun their attack.

"The men are cut up about it,' he said. "They want to send their respects to the family."

I asked if I could speak to the gunners involved, but he said this was not possible. Instead he offered to refer my questions to the men and meet me later in the day.

When I saw him again that afternoon, he had an explanation. He said that the tanks had seen a white pick-up truck with a machine gun mounted on it, flanked by two cars marked as TV vehicles, speeding towards them. The white pick-up had been filled with Iraqis. Through telescopic sights, one of the soldiers had seen an Iraqi put on a gas mask and load the AK47 machine gun. He claimed the Iraqis fired first. "The soldiers thought they were suicide bombers and had stolen the ITN vehicles,' Captain Poland said. "They thought they were being attacked."

The following day, we turned up again, with photographs, and Captain Poland confirmed that one of the men they had seen that day had been Daniel, with his hands up in a position of surrender.

It was the breakthrough we had been looking for. Not only had we learned that American troops were at the scene, but, from Captain Poland, we had the first explanation of why they had fired.

Still, we were no closer to finding out exactly what had happened to Terry. Six weeks after the incident, the trail in Baghdad was cold. So I volunteered to go to Basra to pick up the threads of the investigation.

I drove there with a former SAS soldier in a Jeep and began desperately looking for somewhere to stay. And then came my second bit of luck. We found accommodation with a family living on the outskirts of Saddam's former palace compound.

A young woman in the family not only worked as my translator, but also worked for the Red Crescent. Through speaking to other ambulance drivers, we found the driver of the minibus who had rescued Terry and we arranged to meet at nearby municipal offices.

The driver told me he had gone to the scene that day to look for his son, but instead picked up two wounded Iraqis and Terry Lloyd.

He had seen Terry, wounded but still alive after the initial attack by the Iraqi militia, and told me how he had helped him into the back of his vehicle. "He was badly wounded,' he said. "But then, as we drove away, we came under fire again."

The driver claimed the shots came from a helicopter gunship. He brought the minibus to our meeting and showed me the hole in the back where the bullet entered that he claimed killed Terry. Our ex-SAS man confirmed it appeared to have been fired from above.

It was the first evidence we had that Terry had not died instantly. And ballistics experts have since confirmed that Terry was shot by an American bullet.

The driver also gave me a piece of wood from his vehicle where Terry had been lying. He was convinced that DNA testing would prove his story. I gave it to the Royal Military Police but I never learned what happened to that piece of wood, or if it had indeed been useful.

I also told the ITN team in London, with whom I was speaking daily, about what I had learned. I cried when I thought about the effect it would have on Terry's family, but I was relieved it was not me who had to give them the news.

The situation in Basra was becoming increasingly volatile. It was lawless and terrifying on the streets and we were often forced to abandon our enquiries because of the constant shooting.

Every day we spoke to the Royal Military Police in a desperate bid to convince them to take on our investigation. And every day we trawled the hospitals and morgues, looking for traces of Fred and Hussein, or any witnesses to the events.

As we walked through the corridors of the Az Zubayr hospital, strewn with dazed and confused victims of the conflict, including tiny children bloodied from the deadly blast of cluster bombs, we met a laboratory technician, Mohammed Hussain, who said he knew a man who was at the scene of the attack. Mohammed said we could meet the man at his house in a nearby suburb.

It was daytime, but the location still made me nervous. It was a poor, slum, Shi'ite area known as Ali Baba City because it housed so many thieves and other criminals.

We arrived at a tiny, three-room house where the women were forced to stay in the kitchen and even the children looked at us with some suspicion. At first sight the man, who would give his name only as Kareem, was scary too. Large, overweight and in his early 40s, he was involved with the Iraqi military resistance and seemed to have dressed up for our visit in brown trousers and a neatly pressed matching shirt.

He was cautious at first. But finally, over many meetings, he agreed to tell us his story.

Kareem said he had been in the white Iraqi pick-up truck with the machine gun - which he said had been travelling with the local militia chief to make a phone call - when they stopped the car driven by Fred and Hussein. He claimed they captured them, forcing Hussein on to the back of the pick-up truck and Fred into the back seat.

He said Fred got into the car and was friendly. Terry and Daniel, however, in another car, had got away from them, though he confirmed that the pick-up did fire at Terry's car. This was how Terry had sustained his shoulder wound.

But the pick-up had then come under fire from American tanks. Kareem was shot and wounded but managed to get out of the car and hide. But he gave us detailed information about the other six occupants of the pick-up, including a woman who was a sister of one of the militia.

I met her only briefly, in Basra, but our ex-SAS man later conducted a fuller interview. She explained that she had gone to the burned-out pick-up the next day to collect the remains of her brother, and had buried everything she could find. But she allowed us to exhume the grave for DNA sampling - a task we passed on to the military police. The tests proved a positive match to Hussein, giving credence to Kareem's testimony, and finally telling us what had happened to our translator.

By now it was almost two months since Terry had died and I had begun my investigation. And finally the Royal Military Police agreed to accept my evidence and begin their own investigation.

We had been told by the Government that they would investigate only if they believed a war crime had been committed. Through our detective work, we had forced them to accept this was not only possible but appeared increasingly likely.

It was now May 15, and I was becoming very nervous. Mohammed Hussain had told us he thought we were agents for Israel's Mossad and other rumours were spreading that we worked for the CIA. I'd had enough of bodies and grisly details about what had happened to our ITN team. And I'd had enough of the war.

I also felt our investigation had gone as far as it could - and I was confident that we had uncovered, as fully as we could, what had taken place that day on the road to Basra. Fred Nerac's body has never been found.

On May 16 I flew out of Iraq to London, leaving our evidence with the Royal Military Police. That led to this month's inquest in which the coroner found that Terry had been unlawfully killed.

I don't blame the American troops for firing on the Iraqi pick-up - they genuinely thought they were under attack. I do blame the gunship for firing indiscriminately on wounded civilians trying to escape to safety.

But I also have an incredibly strong sense of achievement that I helped reveal the truth and provide answers for grieving families so desperate to know what had happened to their loved ones.

For me, though, it was time for a career change. I now work in Sydney, Australia, on a current affairs programme.

But if I have a sense of achievement over discovering what happened to Terry and his team, I am also upset that it seems nobody will ever be prosecuted for his death, even though the coroner has recommended it.

The facts are known now. Someone should be held to account. The fact that this looks unlikely to happen makes me very angry.