The parents of a student who was found dead in Russia are calling for the investigation into his death to be reopened in the light of fresh evidence they say shows he was murdered.
Colin Madsen's body was discovered in a Siberian forest on April 4 last year, eight days after he went missing while on a hiking trip with friends.
Local police said he died of hypothermia, having wandered off from the hut where he was staying after taking drugs.
But his parents revealed a new forensics report at a press conference in London on Friday, which they say supports their theory their son's death was not accidental, and that police were involved in a "cover-up".
Colin's American mother, Dana Madsen Calcutt, and his British stepfather, Michael Calcutt, who is originally from Surrey, flew from their home in Missouri to speak as the G20 summit got under way in Hamburg.
They called on Vladimir Putin to direct officials to reopen the investigation into Colin's death and urged Donald Trump to exert any pressure he could on the Russian leader.
University professor Mr Calcutt, 55, said: "We are here to seek justice. We are here to call on the presidents - Putin and Trump - to ensure the case gets reopened."
Mrs Madsen Calcutt said her son, who had been studying Russian in Irkutsk for three years by the time of his death, was an experienced hiker, who knew the Buryatia region well.
He had planned to climb a mountain known as "Love Peak" with three friends - two Russians and an American - when he disappeared from a hut in the village of Arshan on the night of March 27 2016.
His mother flew to the area four days later to look for him, but said she was "interrogated" by police, with questions focusing on why four men were staying together with no women or alcohol.
Colin was found within a mile of the hut and Mrs Madsen Calcutt, a retired doctor, said marks on his face led her to believe he had been suffocated.
"I really, truly feel he was murdered while I was there," she said.
"I do feel the police were involved in a cover-up of the death."
Mrs Madsen Calcutt claimed local police tried to get his friends "to confess to his murder" and even "confess they had sex with his dead body".
"His friends think he was picked up, maybe some sort of confrontation with police and maybe taken up there by horseback," she added.
She returned in July last year, using Russian laws to obtain evidence, including the autopsy video, crime scene photos, drug reports and a 500-page case file.
Inconsistencies included the absence of Colin's necklace, which was handed to her by police, from the crime scene photos, while an American pathologist said the death was "suspicious", she told reporters.
A forensics report carried out by a private US laboratory has now found that the decomposition and position of the body indicate Colin did not die where he was found, while injuries to his body and tearing of his clothing suggest he was "in some sort of fight".
"There is strong support for the hypothesis that the victim was physically abused and died in the process," the report carried out by Independent Forensic Services concluded.
The family's Moscow-based lawyer, Glenn Kolleeny, said there have been hundreds of documented incidents of pre-trial detention in Siberia, and Colin's parents hope their son's case will highlight the issue.
His mother added: "He never liked the spotlight. He was a very, very simple man.
"But I think if his case would help stop this happening, I think he would want that to happen."
Colin's parents have written an open letter to Fiona Hill, the US National Security Council's (NSC) senior director for Europe and Russia, to urge the American president to raise their case with Mr Putin at their planned meeting at the G20 summit.
They said they are confident the new evidence will result in the reopening of the investigation.