Interpol has joined the investigation into the killing of former Russian agent Alexander Litvinenko, the Interfax news agency reported today. The investigation so far has pulled in witnesses in three countries: Britain, where Litvinenko fell sick and died after being poisoned with polonium-210, Russia and Germany. Some of the men who met with Litvinenko before he fell ill travelled from Moscow and Hamburg to London.British investigators yesterday questioned Andrei Lugovoi, a key witness in the killing, in the Moscow hospital where he was undergoing radiation checks, Russian news reports said. Lugovoi told the agencies that he had been questioned for three hours. “I gave testimony exclusively as a witness. I was officially informed of that before the interrogation,” the ITAR-Tass news agency quoted him as saying. “They made no charges against me.”