Federico Borrell García, a young Republican militiaman in the Spanish civil war, died, it now seems certain, on September 5 1936, shot by Francoist rebels on a hillside in Cerro Muriano near Cordoba.
His death might have gone unremarked, except that the image of that moment was celebrated for 40 years as one of the most famous war photographs of the 20th century. It was not Borrell's name that was famous - his identity was established only relatively recently - but that of Robert Capa, whose reputation was made by the photograph. Then, in 1975, came the suggestion that Capa had faked the picture. Now new evidence suggests another, darker twist to the story and adds a new dimension to the complex ethics of reporting war.
The first doubts were raised by journalist Phillip Knightley, in his book on media and propaganda war, The First Casualty, in which he alleged that Capa had staged the scene for the camera. Knightley discovered that the picture had first been published in the French magazine, Vu. The caption, believed to have been written by Capa, described soldiers, "running down the slope. Suddenly their soaring was interrupted, a bullet whistled and their blood was drunk by their native soil."
A year later, Life magazine republished it, captioned as the instant of a soldier's death. Strangely, Capa had shot a picture of a second soldier, similar in appearance, falling on the identical spot; a third picture of a dead militiaman holding his rifle across his stomach corresponds to neither of the first two; and a group shot, in which Borrell is visible, is of a relaxed group posing for the camera. None of the other images suggested a battle was in progress, though the location is known to have been on the frontline.
Capa had been vague about what happened and Knightley had tried in vain to find all the negatives to examine the sequence of events. Capa was a great photographer but he was not averse to faking. In 1937 he fabricated footage for the March of Time newsreel series.
He told the Life photographer, Hansel Mieth, that the Borrell picture had been taken when the militiamen were fooling around, not in the heat of battle as had been believed. She added that Capa seemed upset and said little more except that it "haunted him badly".
Since Borrell's death on that day has been confirmed, the image appears to be that of the moment at which he was shot. But further evidence uncovered by the late curator of the Capa archive, Richard Wheelan - to be shown in a forthcoming exhibition at the Barbican in London - suggests another explanation for Capa's unease.
All the negatives are lost, but the contact prints and Capa's notebooks survive. Wheelan concluded that Capa and his girlfriend Gerda Taro had come across the group of militiamen taking a siesta at the foot of a slope. The siesta was respected by both sides in the war, and since no action was taking place, Capa persuaded the men to pose for a series of simulated scenes. The contact prints of the men pointing their guns over the side of a gully, and of the group cheerfully raising their rifles for the camera appear to confirm this.
The men then climbed a hill, turned and pointed their rifles again; then, in high spirits, ran back down the hill, Capa running beside them, taking pictures. Reaching the gully, they again aimed, and perhaps fired, their rifles. The evidence from other images suggests that the fatal photograph was taken near the edge of the gully. Significantly, a forensics expert consulted by Wheelan categorically asserts that Borrell was not running when he died. He "had been standing flat-footed when he was shot. He was clearly not in stride".
Capa's account, and the Vu caption, stated that the man had been shot as he ran down the hill. Why should Capa have lied? Perhaps for the same reason that he was so fortunately placed right next to Borrell, positioned to take the fatal photograph. If the militia had posed and fired for the camera, they would have attracted the attention of the rebel forces. As Borrell stood to pose for Capa, he was cut down by a rebel bullet.
Was the secret that so tortured Capa the knowledge that without his intervention, Borrell might not have been shot?
Ever since the camera went to war, photographers have staged scenes, rearranged bodies and had events re-enacted for the camera and we look at them in two states of mind - open to their impact as authentic images, and aware that to perceive the camera as a neutral observer is naive. Capa will always be regarded as a great photographer despite the known episodes of fakery, and many curators and critics regard this pre-digital age interrogation of the relationship between photographer and subject as irrelevant. What matters, they argue, is the impact of the image, not what they see as spurious questions of authenticity. But as the story of Capa's iconic photograph shows, discredited images lose their impact. If the story is not what we are invited to believe, we are entitled to resist its effects. If Capa's actions that day did contribute to the death of Borrell, the photograph is telling a radically different and shocking story. Truth and authenticity are not only artistic criteria. They are moral judgments too.
• This is War! Robert Capa at Work opens at the Barbican on October 17 firstname.lastname@example.org