WARNING: CONTAINS DISTURBING IMAGES

The photograph shows three men in suits and a fourth in a long coat, standing by a car at the side of a dirt road on a flat plain panelled with fields. Power lines stretch to the grey horizon. The men could be property developers, surveying the site of a new out-of-town supermarket, but for a single detail: one of them has curled his hand into the shape of a pistol, and he is pointing it at his companions.

The picture, taken in 1963, comes from the archive of the Los Angeles Police Department. The three men in suits are cops, there to recreate the climax of the Onion Field kidnappings, one of the more notorious cases in the history of the LAPD – a police force that, in its 145-year history, has seen more notorious cases than any other. The fourth man (on the far right) is Jimmy Lee Smith, one of the suspects.

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On the night of 9 March, two LAPD officers had been kidnapped in Hollywood and driven north into California's agricultural heartland. One, Ian Campbell, was shot dead in an onion field; the other, Karl Hettinger, ran off into the dark and escaped. The killers, Gregory Powell and Smith, known as "Jimmy Youngblood", were captured the next day. They received death sentences, but evaded execution when California abolished the death penalty several years later.

The photograph has been unearthed for "Unedited!: the LAPD Photo Archives", an exhibition showing at this month's second annual Paris Photo fair in Los Angeles. The fair takes place on the soundstages of Paramount Studios in Hollywood, where some of the shots are being displayed in the shop façades of the studio's New York backlot – a fake Manhattan street – meaning they are "in" LA and New York simultaneously.

A staple of the French capital's cultural calendar for the past 18 years, Paris Photo is the world's biggest photography fair. An American expansion became inevitable, says its director, Julien Frydman, because the US is the world's biggest market for photography. Though most of the fair is devoted to sales by some 150 galleries, Frydman explains, "We also create some shows to nourish the pleasure of the audience, and their understanding of photography."

Hence "Unedited!", which comprises of images culled from a collection of more than a million photos taken by police officers and criminologists over the decades, now stored in LA's City Records Center. Many were once used as evidence by the LAPD Special Investigations k Division, which was set up during the 1920s, making it the oldest crime lab in the US.

Shape Created with Sketch. Unedited!: the LAPD Photo Archives Show all 8 left Created with Sketch. right Created with Sketch. Shape Created with Sketch. Unedited!: the LAPD Photo Archives 1/8 Unedited!: the LAPD Photo Archives 'Chalk outline detailing position of head with knife in hand', 1950 LAPD 2/8 Unedited!: the LAPD Photo Archives 'Morgue, man with floral tattoo', 1945 LAPD 3/8 Unedited!: the LAPD Photo Archives 'Victims feet hanging off bed', 1934 LAPD 4/8 Unedited!: the LAPD Photo Archives 'Shoes, arm and knife', 1950 LAPD 5/8 Unedited!: the LAPD Photo Archives 'Onion Field re-enactment with suspect, Jimmy Lee Smith, and detectives', 1963 LAPD 6/8 Unedited!: the LAPD Photo Archives 'Triptych of images tells a story of suicide’, 1950 LAPD 7/8 Unedited!: the LAPD Photo Archives 'Triptych of images tells a story of suicide’, 1950 LAPD 8/8 Unedited!: the LAPD Photo Archives 'Triptych of images tells a story of suicide’, 1950 LAPD 1/8 Unedited!: the LAPD Photo Archives 'Chalk outline detailing position of head with knife in hand', 1950 LAPD 2/8 Unedited!: the LAPD Photo Archives 'Morgue, man with floral tattoo', 1945 LAPD 3/8 Unedited!: the LAPD Photo Archives 'Victims feet hanging off bed', 1934 LAPD 4/8 Unedited!: the LAPD Photo Archives 'Shoes, arm and knife', 1950 LAPD 5/8 Unedited!: the LAPD Photo Archives 'Onion Field re-enactment with suspect, Jimmy Lee Smith, and detectives', 1963 LAPD 6/8 Unedited!: the LAPD Photo Archives 'Triptych of images tells a story of suicide’, 1950 LAPD 7/8 Unedited!: the LAPD Photo Archives 'Triptych of images tells a story of suicide’, 1950 LAPD 8/8 Unedited!: the LAPD Photo Archives 'Triptych of images tells a story of suicide’, 1950 LAPD

Among the pictures in the exhibition is one from the investigation of the famous "Black Dahlia" case: the gruesome 1947 murder of the young actress Elizabeth Short, whose body had been comprehensively mutilated. Despite (or perhaps even due to) the extensive and sensational newspaper coverage of the case, her killer was never caught. There are images, too, from the investigation of the Manson murders, which brought terror to LA in the summer of 1969.

"The LAPD photographs were originally documents," Frydman says. "But if you select them carefully today, they become a piece of art. Most images we think of when we talk about film noir and the LAPD are images we learnt from the cinema. The images in our mind are not real. So when you look at these photographs, it's hard to decide whether they're real documents or from movies. There's a confusion between fiction and reality. Paramount Studios is a dream location for [the exhibition]: a place that builds images somewhere between fiction and reality."

Thanks to its persistent representation in movies from Chinatown to Beverly Hills Cop (both, incidentally, produced by Paramount), the LAPD is the most famous police force in the world – though at times in its history it has also been infamous. The department suffered for decades from a reputation for institutional racism. The treatment of black civilians by LAPD officers sparked the Watts Riots of 1965 and the Rodney King Riots of 1992. The 1995 prosecution of OJ Simpson fell apart after investigating officer Mark Fuhrman was accused of racism.

The two faces of the LAPD were encapsulated in one man, William H Parker, who served as chief from 1950 to 1966, the longest tenure in the department's history. Parker is credited with modernising the force, but John Buntin, author of the book LA Noir, has described him as "Los Angeles' greatest and most controversial chief of police"; Parker's is the semi-corrupt department portrayed in James Ellroy's LA Confidential.

The relationship between the LAPD and the screen is literalised by one archive image involving Parker on the set of the TV cop series Dragnet in 1963, the same year as the Onion Field incident. Parker, the most famous of all LAPD officers, is seen conversing with Jack Webb, the actor who played Sergeant Joe Friday – the era's most famous fictional cop.

Paris Photo Los Angeles is showing today at Paramount Studios, Hollywood (see parisphoto.com/losangeles for more)