At 2.30am Charlotte Saunders was woken at home in Valencia, Spain, by a telephone call from her sister. Five hours earlier, in London, their brother, Mark, had been shot dead by police.

“I didn't understand what Jenny was saying,” she said. “Then I just remember screaming, 'It's not true'.”

Ms Saunders, 26, still cannot comprehend why her brother, a brilliant barrister and seemingly stable, barricaded himself inside his £2.2 million Chelsea flat five months ago; still less why he started firing a shotgun out of a window to set off a sequence of events that ended with his death.

On that sunny May afternoon, Mr Saunders, 32, came home from work early, poured himself a drink and reached for his licensed shotgun.

Minutes later he was peppering his neighbours' gardens with pellets. One of them said: “He didn't even bother to open the window — he was shooting through the glass.”

Heavily armed police poured into the Kings Road and surrounded the property.

A siege began, with Mr Saunders apparently heedless of requests to negotiate, although his sister insists that he could hear nothing above the noise of police helicopters.

After five hours, seven officers from Scotland Yard's CO19 firearms unit fired 11 shots into Mr Saunders. He died outside his flat soon afterwards.

In her first in-depth interview, Ms Saunders dismisses police claims that her brother committed “suicide by cop”. She denies that he was an alcoholic or a depressive or, as one commentator suggested, an angry man jealous of the professional success of his wife, Elizabeth, a fellow barrister.

“People just make stuff up,” she told The Times, opening a law magazine that said that her brother was “clearly destined for the very top”.

“This is why all this drinking stuff cracks me up,” she said. “You're not tipped to be a High Court judge if you're rolling up pissed every day.”

Sitting on a sofa in her West London home, Ms Saunders said that her brother, who had worked on Chris Tarrant's divorce settlement, was a talented lawyer with a “smart as a whip” sense of humour. He was generous, too, setting up a “fun fund” for her to go on a year abroad.

Many questions remain unanswered but for Ms Saunders one thing is clear: he did not have to die that day. “I question whether they had to kill him,” she said. “I want to know what was in the minds of those officers.”

She points to controversial video footage of the shooting, taken from a police helicopter, that was leaked to the press last month. It seems to show that Mr Saunders was not aiming at officers when he was shot and had not fired his gun for 20 minutes when the police opened fire. It also suggests that he was talking to officers only ten seconds before he was shot.

Astonishingly, Ms Saunders also reveals that a senior member of the Independent Police Complaints Commission (IPCC), which is conducting an investigation into her brother's death, told her that Mr Saunders was “within Taser range” for 20 minutes. If true, it suggests that police might have been able to end the confrontation using non-lethal means.

“Mark was sitting on a windowsill for 20 minutes without a gun,” Ms Saunders said. “If the police were in range for Taser guns why didn't they Taser him then? Why didn't they throw in the CS gas then?”

As with any fatal shooting, an IPCC investigation was started after the death. About 13 investigators have so far collected more than 500 statements and expect to conclude their inquiry in April.

For Ms Saunders, however, the result is a foregone conclusion. She expects the police to be cleared. “Two days after Mark had died a family liaison officer picked me up from Liverpool airport and told me that it was highly unlikely the investigation would come to anything,” she said.

“I thought, 'What's the point of this investigation? You've already made up your mind'.”

Fourteen days later a senior IPCC member repeated the same thing, telling the Saunders family that it was “highly unlikely” that any police wrongdoing would be found, she said. “All through the IPCC spectrum, from the top to the bottom, the consensus was that the inquiry would yield nothing.”

Ms Saunders is terrified that criticising the IPCC will cause them to shut down communication with her. She decided to speak out after her family was subjected to stinging criticism during a landmark court case she brought against the body this month.

During the High Court case, in which Ms Saunders challenged the IPCC decision to allow the officers involved to confer with each other before making their statements, IPCC lawyers accused her family of “aggressive” behaviour in revealing details of the video footage to the press.

“We got blasted in court for leaking confidential information,” she said. “But the IPCC specifically told us it wasn't confidential. Now they're making out that we're unreliable and aggressive but if anyone's been economical with the truth it's them. We don't feel like the victims any more. It's not the way it's supposed to be.”

Ms Saunders accepts that her brother was not entirely innocent but insists that he was not a homicidal maniac as some have painted him. Instead she recalls a man who took up shooting as a sport only because he was a passionate cook.

“He totally loved cooking, so the shooting sourced his cooking,” she said. “People still write on the internet that he had an illegal gun and they don't understand that that was why it was in the house. I don't know whether he got overexcited that day, got his gun out.

“I do think that it started off as a random accident and before he knew it he was boxed in. No one was listening to what he was saying. They wouldn't let him speak to Liz or Mum and he must have felt very abandoned, very trapped.”

In a response to Ms Saunder's allegations, Ian Bynoe, IPCC commissioner, said: “The chair of the IPCC has offered to meet Charlotte Saunders and her family to discuss any concerns or questions they have about the investigation. This would allow us to discuss issues directly with the Saunders family rather than via the media. As yet we have not had a response to this offer but it does still stand.”

A full inquest is expected next year.

Aftermath

May 6 Mark Saunders shot dead at his flat in Chelsea, West London May 7 Independent Police Complaints Commission inquiry begins May 9 Inquest at Westminster Coroner's Court told that the barrister was hit at least five times

July 18 Charlotte Saunders, his sister, wins the right to challenge the IPCC inquiry, saying police conferred on statements

Sept 10 High Court judicial review begins

Sept 11 Ms Saunders claims CCTV showed her brother posed “no risk to the public”

Oct 10 High Court rejects case but says opportunity for police collusion is “institutionalised”

Oct 24 Association of Chief Police Officers changes rules on collusion; now officers generally cannot confer before giving statements

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