THE man convicted of master-minding the 1993 plot to blow up the World Trade Centre in New York, Ramzi Ahmed Yousef, may not know it, but his crime led to the creation of a most unusual photograph. Shortly after the bomb exploded, a photographer from New York city's police department snapped a shot of ground zero, the spot of the bomb's detonation. The resulting colour print depicts a tumultuous yet intricate weave of tangled wires and warped steel—an industrial abstract worthy of Jackson Pollock.

Though the subject matter is brutal (six people were killed and more than 1,000 injured by the blast), the photograph nevertheless exposes and explores the gap between reality and imagination. It is no less than art: a point forcibly made by many of the photographs in the “Police Pictures” exhibition which has just closed in San Francisco and will open at the Grey Art Gallery at New York University on May 19th.

Crime photography has been around almost as long as photography itself. Indeed, the camera and the police detective both arrived in Europe at about the same time, around 1840. Where artists saw photography as a new way to express abstract reality, criminologists seized on the new medium as an unblinking way to document cruel reality. Whereas art photographers began with an idea and then used it to seek, explore or illuminate reality, crime photographers began with reality and often found art. In many crime photographs, even the earliest ones, the eyes of the viewer take in the reality of the horror but they also see the art, the abstraction of human experience that sometimes emerges from even the most horrible scenes.

These chilling photographs, several of them of bloody murders and murderers, confront a vast mythology in attempting to explain what is often inexplicable to most people. An outstanding case in point is the series of stark black-and-white photographs of criminals and crime scenes taken by Weegee in New York during the 1930s and 1940s. With a newsman's Speed Graphic and a Hollywood director's eye, he has strongly influenced the way violent criminals have been conceived of by the public ever since. (The idea of gangsters as portrayed in “The Untouchables” on television owes a lot to him.)

But long before Weegee (whose real name was Arthur Fellig) pandered to popular prejudices about the differentness of criminals, photography was used by criminologists influenced by crude Darwinian ideas about race, heredity and criminality to try to confirm their assumption that villains were somehow physically distinguishable from law-abiding citizens.

In the 19th century, Alphonse Bertillon, a French photographer and the son of a statistical anthropometrist, invented the mug shot to show these differences. He also made a series of measurements of his models' heads and noted characteristic markings. His portraits parlés (speaking likenesses) promoted belief in ridiculous racist theories of human behaviour and the hokum of eugenics.

Bertillon was also the first systematically to photograph crime scenes. He usually did so both at ground level and overhead: “God's-eye view”. But whereas mug shots encourage you to look for differences between yourself and the person in the photograph, Bertillon's turn-of-the-century crime-scene stills uncomfortably reveal similarities. Many of his photographs depict the bloody corpses of women brutally murdered in their parlours, and those parlours look very much like everybody else's. The image provokes inquiry and puzzlement—how did this happen? could it happen to me?—in the same way as some great paintings do.

And even as the early crime photographers sought human differences, the similarities kept cropping up. A photograph of sullen, jut-jawed Lewis Thornton Payne, one of the Lincoln assassination conspirators, taken aboard the ironclad where he was held after his arrest, is nothing short of a proto-Calvin Klein shot. “Marie Cirenchia, Confessed Murderess”, a photograph taken in 1920, shows a pouty, beautiful young woman leaning gracefully—almost innocently yet almost seductively—from the back door of a car (presumably a police car). A page from an album of turn-of-the-century mug shots from Alcatraz reveals a gallery of gentlemen who could all be anybody's uncles or grandfathers.

Published photographs of notorious bandits, such as the James brothers, often glamorised rather than condemned these criminals. Members of the Dalton gang, laid out in caskets after the deadly failure of their bank robbery in Coffeyville, Kansas, still look oddly thrilling. The same is true in a photograph of dead anarchists from the Paris Commune, also in caskets. Bonnie Parker, in a proto-modelling photograph, seems to taunt us from the fender of Clyde Barrow's car. She confidently holds a pistol on her cocked hip and chews defiantly on a cigar. Clyde himself poses in a separate photograph beside his car's grille, with several pistols and rifles artfully arranged in the metalwork.

Indeed, what might be called “gangster chic” crime photographs of the 1930s and 1940s have had a strong influence on fashion and entertainment photography ever since. Many of the fashionable snarls, grimaces and growls that appear in today's music and film magazines could have been lifted from one or other of the FBI “Wanted” posters that have been tacked up in the post offices around America in the past 50 years.

But the exhibition and its accompanying book, “Police Pictures: The Photograph as Evidence” (Chronicle Books; $24.95), also shows how criminal photography can deglamorise crime. Face-down on a bloody white tablecloth at the Palace Bar and Grill in Newark, New Jersey, where he was gunned down, Dutch Schultz is nobody's uncle. Morbid fascination is the only feeling dredged up by the naked, sewn-up body of Baby Face Nelson lying on a coroner's slab (the same slab, incidentally, on which the body of John Dillinger was photographed six months earlier). Al Capone's face-on portrait seemingly confirms what the early crime photographers sought to prove: he just looks like a very bad man.

The ambiguity of the horror, the glamour and the commonplace are not accidental. This exhibition merits that over-used word, important. It forces the visitor to confront the ambiguous feelings most of us have about criminals and criminality.

The photographs on display include a famous shot of the St Valentine's Day massacre and of Bobby Kennedy dying on the floor of the Ambassador Hotel. Among the earliest exhibits are photographs from Alphonse Bertillon's Darwinian experiments; among the latest exhibits a scene-of-the-crime shot of O.J. Simpson's house.

It is remarkable how many of these photographs cross the line from mere documentation into art. Even the semi-formal pre-execution portraits of inmates at the Tuol Sleng death camp, bizarrely ordered by the Khmer Rouge and taken by a teenage photographer, contain a power that goes beyond mere horror. These are the wondering faces of people who obviously have no idea what has happened to them, or what is going to happen, or why—in other words, the faces of all of us.