The European Court of Human Rights found on Tuesday that Russia was responsible for the assassination of ex-KGB officer Alexander Litvinenko, who died an agonizing death in 2006 after being poisoned in London with a rare radioactive substance.

Litvinenko, a defector who had become a vocal critic of the Kremlin, died three weeks after drinking green tea laced with polonium-210 at a plush London hotel.

Britain has long blamed the attack on Moscow, and the European court in Strasbourg, France, agreed on Tuesday, saying in a statement that "Mr. Litvinenko’s assassination was imputable to Russia," prompting a swift rebuke from the Kremlin.