Evidence of half billion-dollar transactions, threats and links to a high-profile money-laundering case were lost by police investigating the death of a wealthy Russian, an inquest in London has heard.

A civilian police translator trawled through documents, text messages and Skype chats taken from the computer of Alexander Perepilichnyy in December 2012 after his death the month before.

Russian-born Ekaterina Clarke-O’Connell provided a snapshot of the lost evidence to the inquest, before the coroner, Nicholas Hilliard QC, at the Old Bailey, which is examining whether the 44-year-old businessman was murdered or died of natural causes.

She concluded that something was “not right”, as she recalled massive sums of money – up to $500m (£310m in November 2012) – flowing through accounts, the Old Bailey heard.

At the time he collapsed while jogging near his home in Weybridge, Surrey, he was helping the UK-based investment fund Hermitage Capital, co-founded by the financier Bill Browder, to expose a $230m (£142m in November 2012) money-laundering operation.

Timeline Poisoned umbrellas and polonium: Russian-linked UK deaths Show Georgi Markov In one of the most chilling episodes of the cold war, the Bulgarian dissident was poisoned with a specially adapted umbrella on Waterloo Bridge. As he waited for a bus, Markov felt a sharp prick in his leg. The opposition activist, who was an irritant to the communist government of Bulgaria, died three days later. A deadly pellet containing ricin was found in his skin. His unknown assassin is thought to have been from the secret services in Bulgaria. Alexander Litvinenko The fatal poisoning of the former FSB officer sparked an international incident. Litvinenko fell ill after drinking tea laced with radioactive polonium. He met his killers in a bar of the Millennium hotel in Mayfair. The pair were Andrei Lugovoi – a former KGB officer turned businessman, who is now a deputy in Russia’s state Duma – and Dmitry Kovtun, a childhood friend of Lugovoi’s from a Soviet military family. Putin denied all involvement and refused to extradite either of the killers. German Gorbuntsov The exiled Russian banker survived an attempt on his life as he got out of a cab in east London. He was shot four times with a silenced pistol. He had been involved in a bitter dispute with two former business partners. Alexander Perepilichnyy The businessman collapsed while running near his home in Surrey. Traces of a chemical that can be found in the poisonous plant gelsemium were later found in his stomach. Before his death, Perepilichnyy was helping a specialist investment firm uncover a $230m Russian money-laundering operation, a pre-inquest hearing was told. Hermitage Capital Management claimed that Perepilichnyy could have been deliberately killed for helping it uncover the scam involving Russian officials. He may have eaten a popular Russian dish containing the herb sorrel on the day of his death, which could have been poisoned. Boris Berezovsky The exiled billionaire was found hanged in an apparent suicide after he had spent more than decade waging a high-profile media battle against his one-time protege Putin. A coroner recorded an open verdict after hearing conflicting expert evidence about the way he died. A pathologist who conducted a postmortem examination on the businessman’s body said he could not rule out murder. Scot Young An associate of Berezovsky whom he helped to launder money, he was found impaled on railings after he fell from a fourth-floor flat in central London. A coroner ruled that there was insufficient evidence of suicide. But Young, who was sent to prison in January 2013 for repeatedly refusing to reveal his finances during a divorce row, told his partner he was going to jump out of the window moments before he was found.

Skripal poisoning Sergei Skripal and his daughter Yulia were were found unconscious on a bench in the Maltings shopping centre in Salisbury after 'suspected exposure to an unknown substance' which was later identified as chemical weapon novichok. In the aftermath Theresa May blamed Vladimir Putin and expelled 23 Russian diplomats who were suspected of spying. Two Russian men using the identities Ruslan Boshirov and Alexander Petrov were named as suspects. They appeared on Russian TV to protest their innocence.

The Skripals survived. However a local woman, Dawn Sturgess, died after spraying novichok on her wrists from a fake Nina Ricci perfume bottle converted into a dispenser, which had been recovered from a skip by her partner Charlie Rowley.

A month before his death, Perepilichnyy had also fought off a legal challenge by a debt recovery firm allegedly led by the prime suspect in the Alexander Litvinenko poison case.

Lucas Fear-Segal, representing Perepilichnyy’s life insurers Legal and General, told Clarke-O’Connell: “Essentially, the original documents you looked at have gone missing.

“It’s either the fault of Surrey police or south-east counter-terrorism unit, and backup cannot be accessed by Surrey police so all we have got is you.”

Recalling the documents, the witness said: “They were work-related to the deceased. We were given trigger words that could flag up threats, anything unusual, money-laundering activities, some names.

“It seems like something was missing. The files were presented to us in chaotic order so it could be just a random picture.”

Asked iwhether she found evidence of money laundering, Clarke-O’Connell said: “I have seen large sums of money going in and out of different accounts ... If you think to see something suspicious, these large sums of money, as a human, you think something is not right.

“My job was to flag up something mentioning large sums of money, which I did.”

She found three threatening text messages, with the first saying in Russian: “Alexander you will go to prison for a long time” and demanding he pay the equivalent of £6,000 in roubles.

The next text told him in Russian not to “do anything silly because we have everything under control”.

A text message in English from Perepilichnyy’s wife, Tatiana, said someone called Rishat was “milking him like a cow – he betrayed you a long time ago”.

Fear-Segal asked Clarke-O’Connell whether she had not figured out that Rishat was Rishat Ismagilov, Perepilichnyy’s brother-in-law who was said to be involved in a Russian money-laundering vehicle.

He also suggested the evidence could show Perepilichnyy’s business being “controlled by some sort of extortionist”.

The court heard the lost documents may have illustrated how Perepilichnyy was trying to minimise his role in the money-aundering case and reach agreement on his evidence to Swiss authorities.

His Skype messages referred to Hermitage, Swiss authorities and money laundering and having “all the documents to prove this”.

On 13 October, 2012, Perepilichnyy made an internet search for an article on Sergei Magnistky, the late Russian lawyer who helped uncover the Hermitage fraud.

Clarke-O’Connell confirmed she found nothing on the computer about Perepilichnyy’s health being poor or of concern.

John Beggs QC, for Surrey police, complained the witness was giving an “inexpert” opinion on what was “fragmentary evidence”.

Investigating officer DS Ian Pollard said Surrey police was given two discs but the computer data was not on them.

Investigators were unable to obtain a backup copy from Sectu.