Is Russian crime arriving on UK shores?
Russian money has poured into London, but it is feared organised crime is accompanying it. Panorama investigates a death in a Russian prison that has brought the threat of violence to the UK. Could a whistleblower found dead on the streets of Surrey be the latest victim of the Russian crime wars?
When you investigate organised crime in Russia, strange things happen.
We had almost finished filming when one of our team received an email in Russian from a man calling himself "H". He said he was a hacker and had been approached by an agent of the Russian state to hack into BBC emails. He had apparently decided to warn us instead.
We had been looking into one of the most notorious organised crime cases in Russia - the theft of $220m from the Russian Treasury in 2007.
The case has become an international scandal and London is at its centre because the man who blew the lid on it is a UK citizen living in the capital.
Bill Browder used to manage billions of dollars of investments in Russia through his company Hermitage Capital. But in 2007, the offices of Hermitage and its lawyers were raided by the Russian Interior Ministry.
A few months later, Hermitage discovered that three of its companies had been stolen. The three companies were then used to commit a massive tax fraud in Russia totalling $220m.
In Moscow, a legal adviser called Sergei Magnitsky had begun to investigate for Browder, but within months Magnitsky was arrested on suspicion of tax offences.
For nearly a year, Magnitsky was held in pre-trial confinement. During that time he developed pancreatitis, and on 16 November 2009, he died.
The Russian authorities said he died of heart failure, but a report by the Kremlin's own Human Rights Council concluded he had been denied medical treatment and his right to life had been violated.
Browder is convinced Magnitsky was silenced and is now lobbying governments to impose legal sanctions against the people he says were involved.
Browder's activities have brought threats of violence to his staff in the UK, but he has refused to back down, releasing online videos naming Russian officials linked to the Magnitsky case.
Weeks after the first video was released, Hermitage lawyers were contacted by a man claiming to have inside information about the fraud.
Alexander Perepilichny handed over confidential bank documents, showing money transfers into Swiss bank accounts linked to some of the Russian officials who had helped approve the fraudulent tax refund.
Hermitage submitted a criminal complaint to the Swiss authorities who launched a major investigation. They had meetings with Perepilichny and froze bank accounts. Nineteen months later, Perepilichny was found dead on an exclusive street in Surrey.
Now, more than five months since Perepilichny's death and after two post mortems, Surrey Police says it still does not know how he died. Many people are wondering if he was murdered.
We wanted to find out more about Perepilichny. While in Moscow, we tracked down several of his former business associates. Some agreed to speak to us, but not on camera. They were too nervous.
More than once, we heard that Perepilichny's name was on a hit list and that he was living in the UK because of fears for his safety.
If Perepilichny was killed - and it is a big 'if' - our snooping around in Moscow will certainly have caused alarm in some quarters - perhaps enough to set a hacker on our case. But Perepilichny's enemies were not the only target we were chasing in Moscow.
Roman Anin is a journalist with the Russian newspaper Novaya Gazeta and the Organized Crime and Corruption Reporting Project (OCCRP).
He showed us documents which suggest that millions of dollars of the Russian fraud money were moved into UK-registered companies. All of the UK companies that we looked at had bank accounts in Eastern Europe and have now been dissolved.
But what is striking is that the millions of dollars that they were receiving, according to the documents, do not show up in their accounts.
We showed our evidence to Andrew Mitchell QC, one of the UK's leading experts on money laundering. He told Panorama: "They indicate that somebody has used UK corporate entities to launder the proceeds of crime in Eastern Europe."
Not surprisingly, the Hermitage case has heightened fears about Russian criminality in the UK. Some are calling for the UK government to introduce a "Magnitsky Act," which would make it harder for corrupt foreign officials and human rights abusers to come to the UK.
Dominic Raab MP is leading the campaign. "We don't want Britain to become a playground for these gangsters, let alone a battleground for the violence that tends to follow," he said.
The UK government says it is seriously concerned about the death of Sergei Magnitsky and has asked Russia to investigate his death properly.
However, it has so far resisted calls for a Magnitsky Act, saying it already has the power to keep human rights abusers out of the UK. Critics say it is simply unwilling to upset Putin's Russia, no matter what happens on the streets of Britain.
Panorama: The Russians Are Coming, BBC One, Monday 29 April at 20:30 BST and then available in the UK on the BBC iPlayer.