"(Russian) Interpol has been engaged in the investigation for the sake of exchanging information with certain countries, as an information channel," said Tatyana Trunayeva, spokeswoman for Interpol's Russian branch.
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 on Nov. 1 — the day he is believed to have fallen ill — traveled to London from Moscow and Hamburg.
The involvement of Interpol, the largest international police organization, should ease the way for investigators in each country to obtain evidence and testimony. Interpol, which has 186 member nations, coordinates cross-border police cooperation.
Trunayeva said the mission of the Russian branch of Interpol in the probe was to facilitate information exchange.
British investigators on Monday 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."
Lugovoi said the results of his medical tests would be known later this week but said he was "unlikely" to make them public.
German authorities, meanwhile, found traces of polonium-210, the rare radioactive substance that killed Litvinenko, in locations visited by Dmitry Kovtun, another Russian contact, before his Nov. 1 meeting with Litvinenko. The Russian Prosecutor General's office said Kovtun had been diagnosed with radiation poisoning.
Lugovoi told the RIA Novosti news agency Monday that Kovtun was in stable condition and "feeling normal."
Russian and British investigators interviewed Kovtun last week before German investigators found traces of polonium-210 in Hamburg, the city where he spent four days immediately before meeting with Litvinenko.
Traces of polonium-210 have now been confirmed in a Hamburg apartment of Kovtun's ex-wife, where he spent two nights, and the car that picked him up from the Hamburg airport when he arrived from Moscow.
On Monday, police said that the ex-wife, her partner and two small children were taken to a Hamburg hospital for tests to check whether they absorbed any radiation.
The family showed no signs of external contamination, prompting tests to determine whether they ingested a radioactive substance.
German prosecutors are investigating Kovtun on suspicion that he may have illegally handled radioactive material. They have left open whether the radioactive trail meant he might have been involved in Litvinenko's poisoning, saying that he may have been a victim or could have been involved in procuring the polonium.
Lugovoi, Kovtun and a third associate who was in London with them on the weekend of Nov. 1, Vyacheslav Sokolenko, have denied involvement in Litvinenko's death.
Litvinenko died in London on Nov. 23, blaming the Kremlin for poisoning him — an accusation that Russian officials vehemently deny.
Deputy Kremlin spokesman Dmitry Peskov told the British Broadcasting Corp.'s "Newsnight" program that it was "unthinkable that the Russian government can be behind any killing."
Asked if rogue elements of the Russian security service could be responsible, he said: "This is a question to investigators, I would rather exclude such a possibility.
Peskov reaffirmed that Scotland Yard investigators in Russia weren't allowed to question witnesses face to face and had to rely on their Russian counterparts for assistance.
"Can you imagine Russia's agents coming to London here, and questioning anyone they want? It's unimaginable," Peskov said.
In a separate development Monday, a French police report obtained by The Associated Press said Yevgeny Limarev, who reportedly blew the whistle on Russian agents' plans to target Litvinenko shortly before he was poisoned, has gone missing from his home in the French Alps with his wife and teenage daughter.
Mario Scaramella, an Italian security expert who met with Litvinenko at a London sushi bar later Nov. 1, reportedly said he had shown him materials from Limarev suggesting Russian agents' involvement in the October killing of Russian investigative journalist and Kremlin critic Anna Politkovskaya. The materials also reportedly indicated that the Honor and Dignity group of Russian security veterans was plotting to kill Litvinenko and other Kremlin critics.
The group's head, Valentin Velichko, has rejected the allegations as nonsense.
Limarev, whom Russian newspaper Izvestia reported is close to self-exiled tycoon and Kremlin foe Boris Berezovsky, who lives in Britain, has not been seen in his home in Cluses, France, since Friday.