Viewers tuning into a Channel 4 documentary about the death of the late Princess of Wales were left unimpressed by the top female detective who led the investigation into the 1997 Paris crash.

Brigade Criminelle chief Martine Monteil, who was first to arrive on the scene 25 years ago, told Channel 4’s Investigating Diana: Death in Paris how police found broken car bits, evidence of braking and traces of paint on the vehicle carrying Diana and her lover, Dodi Fayed.

During the latest episode of the four-part documentary series, Monteil, who is now retired from the police force, tells the programme that the French police department charged with handling the aftermath of the royal’s death were forced to run ‘the most expensive investigation in the world’.

Chief detective called Diana Fiat Uno investigation ‘a huge pressure’

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This year marks the 25th anniversary of Princess Diana’s death in Paris on August 31, 1997. Channel 4’s documentary Investigating Diana: Death in Paris started last night but viewers weren’t impressed by the way French police came across. (Pictured: The Princess of Wales in a red and black Catherine Walker evening dress during an official visit to Paris in November, 1988)

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Brigade Criminelle chief Martine Monteil, who was first to arrive on the scene 25 years ago, was deemed ‘untrustworthy’ by some viewers of the first episode of the new documentary. She has since retired from the police force

Operation Paget, carried out by British police, concluded that Diana’s death was a ‘tragic accident’. There was, Lord Stevens concluded, ‘no evidence’ of a murder conspiracy or a cover-up by MI6 (Pictured: Wreckage of the car Diana died in after it crashed in the Alma Underpass, in Paris, being lifted onto a truck)

However, many of those watching the programme took to social media to say they found Monteil ‘untrustworthy’, calling the French police investigation flawed.

Monteil says searching for a white Fiat Uno – believed to be carrying paparazzi chasing Diana’s car – ‘tied up’ a huge number of police officers, making their job ‘hell’.

The detective said: ‘We were accused of running the most expensive investigation in the world but we didn’t have a choice.’

One viewer, Karen Wooley, wasn’t convinced by the detective’s account, writing online: ‘I’m not a conspiracy theorist but everything the female French head detective says seems a lie and she always covers her mouth when she makes a closing statement #bodylanguage.’

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Monteil told the documentary she became ‘obsessed’ with finding even the tiniest piece of evidence and her team was forced to carry out an expensive investigation

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Viewers were quick to suggest Monteil had something to hide. One viewer wrote online: ‘I’m not a conspiracy theorist but everything the female French head detective says seems a lie and she always covers her mouth when she makes a closing statement #bodylanguage’

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Another added: ‘Anyone else find the French female detective really untrustworthy? I genuinely can’t believe a word she says’

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Monteil spoke to the documentary a quarter of a century after the crash, which French police determined to be an accident, took place

Another Twitter user said: ‘Anyone else find the French female detective really untrustworthy? I genuinely can’t believe a word she says.’

Yet Monteil wasn’t the only person to receive criticism from viewers, with many suggesting her force in general hadn’t carried out the investigation well.

Twitter user Tom Artz said: ‘French police are like Parisian waiters — rude, officious and arrogant.’

Meanwhile, another even likened it to the bungling investigation by Portuguese police into Madeleine McCann.

Social media user Adam added: ‘French police don’t come out of this looking good.

‘The pathetic cowards closed ranks with the British establishment.

‘You would think after 25 years they would have perfected their acting skills.’

French detectives reveal pictures taken seconds before Diana’s death

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Viewers flocked online to criticise Monteil and her team. Twitter user Tom Artz said: ‘French police are like Parisian waiters — rude, officious and arrogant’. Social media user Adam added: French police don’t come out of this looking good. You would think after 25 years they would have perfected their acting skills’

Monteil told the documentary she became ‘obsessed’ with finding even the tiniest evidence, like the pearls, because it was all ‘important’ when probing what happened in the Pont de l’Alma tunnel on August 31, 1997.

‘We started to find these little clues,’ Monteil said of the initial crash scene in the documentary.

‘I was obsessed with finding things because it’s important.’

‘We saw signs of braking. Pieces of red light from another car. On the side of the car were traces of paint.’

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The wreckage of Princess Diana’s car is lifted on a truck in the Alma tunnel of Paris on August 31, 1997. The royal died a few hours after the crash at Paris hospital of La Pitie-Salpetriere of her injuries

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Brigade Criminelle chief Martine Monteil, who was first to arrive on the scene, recalled how they found ‘tiny pearls,’ broken car bits, evidence of braking and traces of paint on the vehicle carrying Diana and her lover, Dodi Fayed

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Brigade Criminelle officer Eric Gigou shared how police took the paparazzi into custody and used their photos to piece together the moments before and after the crash

Monteil added: ‘I even found some tiny pearls. They belonged to the Princess.’

The documentary covers the dual investigation into Diana and Dodi’s deaths, the initial 1997 inquiry by the French Brigade Criminelle and Operation Paget, as well as the inquest into the pair’s deaths. It features interviews with both the French and Metropolitan police forces.

It also probes the Scotland Yard chiefs’ handling of a note detailing Diana’s fears she would be killed in a staged car accident.

Diana voiced her fears to her lawyer, Lord Mishcon, in October 1995. After Diana’s death, he passed his contemporaneous typed account of their meeting to senior Metropolitan Police officers who put it in a safe. But the note was not passed to French authorities investigating her crash for six years.