The killing of Archbishop Oscar Romero was one of the most notorious crimes of the cold war. Was the CIA to blame?

San Salvador. In the bright morning sunlight of March 24 1980, a car stopped outside the Church of the Divine Providence. A lone gunman stepped out, unhurried. Resting his rifle on the car door, he aimed carefully down the long aisle to where El Salvador's archbishop, Oscar Arnulfo Romero, was saying mass. A single shot rang out. Romero staggered and fell. The blood pumped from his heart, soaking the little white disks of scattered host.

Romero's murder was to become one of the most notorious unsolved crimes of the cold war. The motive was clear. He was the most outspoken voice against the death squad slaughter gathering steam in the US backyard. The ranks of El Salvador's leftwing rebels were being swelled by priests who preached that the poor should seek justice in this world, not wait for the next. Romero was the "voice of those without voice", telling soldiers not to kill.

The US vowed to make punishment of the archbishop's killers a priority. It could hardly do otherwise as President Reagan launched the largest US war effort since Vietnam to defeat the rebels. He needed support in Washington, which meant showing that crimes like shooting archbishops and nuns would not be tolerated.

The ordering of the murder was blamed on the bogeyman of the story, a military intelligence officer called Major Roberto D'Aubuisson who had, conveniently for Washington, recently left the army. In the weeks before the murder, he was repeatedly on television using military intelligence files to denounce "guerrillas". Those he accused were often murdered. Romero was near the top of the list.

But US promises to bring justice came to nothing. With no trigger-man, gun or witnesses, officials claimed lack of evidence. D'Aubuisson went on to become one of El Salvador's most successful politicians before throat cancer killed him at the end of the civil war 12 years later - the revenge of God, many concluded.

However, new evidence suggests that Washington not only knew far more about the killing than it admitted - but also did nothing to investigate for fear of jeopardising its war effort. Vital evidence was ignored. Key witnesses, including the most likely gunman, were killed by those supposed to be investigating.

Seven years and 50,000 deaths after Romero's murder, I was feeling out of my depth as a novice reporter sitting on a park bench talking to a young deserter from Major D'Aubuisson's death squads who called himself Jorge. In 1984, he told me, he had guarded a safe house for three former guerrillas nicknamed the Little Angels. They spent their days with prostitutes, smoking marijuana and playing rock music at full volume. At night, they sallied out to capture and kill their former rebel comrades.

The men received orders from a National Police detective, Oscar Perez Linares, who came to the house. A man of few words, he was treated by the others with the respect reserved for those not afraid to kill. Several times, Jorge heard the others laughing at how Linares had shot Romero. Linares sat with a half-smile. "You should have seen the blood that came from that priest!" was his only comment.

At the time, Jorge's story was impossible to check - although the rest of what he said turned out to be true. After the war, I looked in declassified CIA files. Sure enough, in mid-1983, an unusually detailed CIA report, quoting a senior Salvadoran police source, named Linares as a member of a four-man National Police squad which murdered Romero. Other Salvadoran officers said the same thing. And the man who drove the car which took the killer to the church also picked out a photo-fit of Linares. So why, if the CIA had such evidence, and solving the murder was such a priority, was nothing done?

Linares was not just any detective. Nor were the Little Angels simple ex-guerrillas. Before the war, when D'Aubuisson ran El Salvador's intelligence agency ANSESAL - set up by the CIA to guard against Communist infiltration of its hemisphere - Linares was one of his most successful agents, infiltrating the fledgling rebel groups. One of his best spies inside rebel ranks was the head of the Little Angels, who Jorge knew as El Negro.

After the war, I tracked down El Negro's former rebel comrades. He had been a top guerrilla leader, trained in Cuba. But he was, they said, a traitor. In the years following Romero's murder, he helped to systematically destroy the rebels in western El Salvador, where thousands of leftist sympathisers were murdered. He betrayed gun-running units in neighbouring countries and led guerrilla columns into ambushes. The operation continued even after 1982, when the rebels realised El Negro was a traitor and he fled their ranks. For the next three years, he and Linares led police units which dismantled the rebels in the capital. Some rebel survivors described how El Negro would appear during torture sessions. "Remember the time we were together in Cuba?" he would say.

This was not a death squad rabble, but rather a highly structured and effective dirty war in which the Americans were closely involved. The much-blamed death squads, like the White Hand and the Secret Anti-Communist Army, never existed as more than exotic names used to sow terror. Military and police intelligence, as CIA reports from 1983 make clear, did the killing. The CIA should have known. It helped the police throughout this period, contracting in urban intelligence experts.

"They would sit next door and write out the questions, leaving it up to the Salvadorans to extract the information," said Gerardo Le Chevalier, a senior Salvadoran official at the time. Neither was Major D'Aubuisson the boss. He played the bogeyman very well, leading political rallies to sing his party's anthem.

"El Salvador will be the tomb where the reds end up!" he would shout. But the major always denied his organisation was behind all the bloodshed. "Who do you think I am, Superman?" he barked at us when confronted with his death squad record. And, indeed, far too many units were involved for the major to have had control. It was rather the policy of the generals and colonels at the top of the army. In 1980, when Romero was murdered, the major was coordinating closely with the small clique of rightwing generals and colonels struggling to keep the army under their control. D'Aubuisson may have ordered the killing. But it is very unlikely he did so alone.

Several US military men told me they could not get rid of the senior officers with the worst human rights records without risking the whole war. The US had no access to spies like El Negro Mario. The Salvadorans, who never really trusted their US backers, kept that for themselves. Linares and the Little Angels moved from safe house to safe house. US officials said that to clean up the army would have fatally demoralised it. More to the point, it would have lost its eyes and ears.

By late 1983, however, the dirty war was running out of steam. The army was now losing a conventional war in the mountains - against well-armed guerrillas who could not be arrested and murdered in the middle of the night. Reagan desperately needed Congress to grant more military aid, which meant cleaning up the Salvadoran army's image.

Vice President George Bush and his aid Lt Col Oliver North were dispatched to El Salvador. Later Bush claimed he faced down the death squads. In fact, he met with the Salvadoran high command. Afterwards, North slipped them a list of nine names the Americans wanted removed from the army. Linares was one of the names.

His end came three years later, in 1986. By now, he and the Little Angels were kidnapping rich businessmen who refused to pay towards keeping the clandestine intelligence operation going. The kidnappers would pretend to be guerrillas. But some victims recognised the voices of army officers talking outside their rat-infested cells. The FBI and the National Police were sent to investigate.

Linares, El Negro Mario and the others were quickly identified. Linares was arrested after fleeing to Guatemala by a special police unit the Americans set up to investigate human rights crimes - particularly Romero's murder. They killed him on the way back to San Salvador. El Negro Mario was also captured and killed. "They knew far too much to live," was the verdict of one top Salvadoran officer.

The details of Romero's murder, as well as thousands of others, went to the grave with them.