Informant38

the situation in Russia that has allowed this to occur:

Some Russian media claimed that it was Boris Berezovsky who organized the murder of Sergei Yushenkov through his agent Mikhail Kodanev.
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He also tried to investigate criminal activities of FSB officers related to money laundering through the Bank of New York and illegal actions of Yevgeny Adamov, a former Russian Minister of Nuclear Energy. This case was under the personal control of President Vladimir Putin. In June of 2003, Shchekochikhin contacted the FBI and got an American visa to discuss the case with US authorities. However, he never made it to the USA because of his sudden death.
[...]
Shchekochikhin died suddenly on July 3 2003 after a short and mysterious illness. His relatives were denied an official medical report about the cause of his illness, and were forbidden to take specimens of his tissues to conduct an independent medical investigation. Nevertheless, they managed to send a specimen of his skin to London, where a tentative diagnosis was made of poisoning with thallium...
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Politkovskaya
Yushenkov
Shchekochikhin

wikipedia
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Berezovsky and Abramovich are Russian "oligarchs" living in London.