London (CNN)Nikolai Glushkov, a Russian businessman the UK refused to extradite to Russia on fraud charges, has died in his London home, his neighbor Pat Egan told CNN. According to Egan, police knocked on her door in Kingston, southwest London, at 3.30 a.m. local time to ask whether she or her husband had seen or heard anything. In a statement, British police confirmed they were investigating the death of a man in his 60s in southwest London, but did not identify him. Police said the death was being treated as unexplained, adding that the counterterrorism team would lead the investigation “as a precaution because of associations that the man is believed to have had.” “There is no evidence to suggest a link to the incident in Salisbury,” police added, referring to the poisoning of former Russian spy Sergei Skripal and his daughter Yulia. Egan said forensic crews attended the scene, but were not wearing hazmat suits. She added that police removed several bags of evidence. According to the Russian Embassy in London, the UK refused to extradite Glushkov in February 2016 on charges of large-scale fraud and embezzlement during his time as the deputy director of the Russian national airline Aeroflot.The émigré was acquainted with Russians who died in unexplained circumstances in the UK. Glushkov was questioned as part of a British inquiry into the poisoning of Alexander Litvinenko, who died a slow and painful death after drinking tea laced with highly radioactive polonium-210 in 2006 in a hotel bar in London. Glushkov told the inquiry that he bumped into Litvinenko in the office of Boris Berezovsky, a powerful oligarch and émigré who was found dead in 2013 on the bathroom floor of his home in Berkshire, on the western outskirts of London. British police said at the time they found no sign of a struggle, suggesting Berezovsky had taken his own life. Glushkov was a friend of Berezovsky and argued publicly that he believed the exiled tycoon had been murdered. Glushkov won notoriety in Russia’s bare-knuckled transition to a market economy in the 1990s, when business, political and criminal interests often collided amid the privatization of state assets. Boris Berezovsky was found dead on the bathroom floor of his home in 2013. Glushkov was formerly an employee of Berezovsky, one of the billionaires who bankrolled the 1996 re-election of President Boris Yeltsin. Berezovsky subsequently fell out with Yeltsin’s successor, President Vladimir Putin, and Glushkov found himself caught up in a politically charged corruption case. In 2000, Glushkov was arrested on charges of embezzling funds from Aeroflot, in which Berezovsky was a shareholder. Glushkov was sentenced in 2004 by the Savelovsky Court of Moscow to three years and three months in prison and released for time served, Russian news agency Tass reported. According to Tass, he received political asylum in the UK in 2010. But Russian courts continued to pursue him. Last year, a Moscow court tried him in absentia for embezzlement from Aeroflot,the Russian court-reporting agency RAPSI reported.