Background: Alexander Valterovich Litvinenko (Russian: Aleksandr Val'terovich Litvinenko, IPA: [aljI'ksandr 'valtir@vjItc ljItvjI'njenk@]; 30 August 1962 (Or 4 December 1962 by father's account - 23 November 2006) was a British naturalised Russian defector and former officer of the Russian FSB secret service who specialised in tackling organised crime. According to US diplomats, Litvinenko coined the phrase Mafia state. In November 1998, Litvinenko and several other FSB officers publicly accused their superiors of ordering the assassination of the Russian tycoon and oligarch Boris Berezovsky.
Context: On 1 November 2006, Litvinenko suddenly fell ill. On 3 November, he was admitted to Barnet General Hospital in London. He was then moved to University College Hospital for intensive care. His illness was later attributed to poisoning with radionuclide polonium-210 after the Health Protection Agency found significant amounts of the rare and highly toxic element in his body.  In interviews, Litvinenko stated that he met with two former KGB agents early on the day he fell ill - Dmitry Kovtun and Andrei Lugovoy. Though both denied any wrongdoing, a leaked US diplomatic cable revealed that Kovtun had left polonium traces in the house and car he had used in Hamburg. The men also introduced Litvinenko to a tall, thin man of central Asian appearance called 'Vladislav Sokolenko' who Lugovoy said was a business partner. Lugovoy is also a former bodyguard of Russian ex-Acting Prime minister Yegor Gaidar (who also suffered from a mysterious illness in November 2006). Later, Litvinenko had lunch at Itsu, a sushi restaurant in Piccadilly in London, with an Italian acquaintance and nuclear waste expert, Mario Scaramella, to whom he made the allegations regarding Italy's Prime Minister Romano Prodi. Scaramella, attached to the Mitrokhin Commission investigating KGB penetration of Italian politics, claimed to have information on the assassination of Anna Politkovskaya, 48, a journalist who was killed at her Moscow apartment building in October 2006.  Marina Litvinenko, his widow, accused Moscow of orchestrating the murder. Though she believes the order did not come from Putin himself, she does believe it was done at the behest of the authorities, and announced that she will refuse to provide evidence to any Russian investigation out of fear that it would be misused or misrepresented. On a court hearing in London in 2015, a Scotland Yard lawyer concluded that "the evidence suggests that the only credible explanation is in one way or another the Russian state is involved in Litvinenko's murder".
Question: why was he poisoned?. Whats the answer?
Marina Litvinenko, his widow, accused Moscow of orchestrating the murder.