Mario Scaramella, the Italian who met Alexander Litvinenko at a sushi bar on the day the former Russian spy was poisoned with Polonium 210, was arrested on his return to Italy from London on Christmas Eve.
Mr Scaramella was arrested at Naples airport, driven to Rome and locked up in Regina Coeli jail. A police spokesman said that he may be charged with arms trafficking and slander but would be interviewed by a magistrate tomorrow.
Scotland Yard said Mr Scaramella's arrest had no connection with the former-KGB agent's death. The Italian has said that he set up the rendezvous to show Mr Litvinenko emails warning that both men were being targeted by Russian assassins.
Mr Litvinenko initially blamed Mr Scaramella for poisoning him, and Russian and Chechen websites identified him as the guilty party. But although British police questioned him thoroughly, his name has never figured on an official list of suspects.
Mr Scaramella's sudden notoriety in connection with Mr Litvinenko has thrown a harsh spotlight on his activities in Italy, however. The 36-year-old Neapolitan became well known in the past five years as an expert on Russian intelligence for a parliamentary commission investigating KGB interference in Italy. Set up by then-prime minister Silvio Berlusconi supposedly to probe the archives of KGB double agent and defector Vasili Mitrokhin, the Mitrokhin Commission was mostly involved in seeking communist-related dirt to fling at Mr Berlusconi's political adversaries.
Mr Scaramella was appointed consultant to the commission by its president, Paolo Guzzanti, a senator in Mr Berlusconi's Forza Italia party and deputy editor of Il Giornale, the daily paper owned by Mr Berlusconi's brother.
Mr Scaramella was useful to Mr Guzzanti because he had numerous acquaintances among KGB and FSB defectors in the West and an apparently inexhaustible knowledge of the minutiae of espionage intrigues in the former-Soviet Union.
Among his theories is a claim that a Soviet warship planted 20 nuclear torpedoes on the floor of the Bay of Naples.
But the information he provided to the commission seemed to some of its members to be as dubious as the Neapolitan's own credentials. Although he likes to be called "professor", none of Mr Scaramella's numerous claims of connection to universities seem to hold water.
The accusation of arms smuggling appears to relate to another of Mr Scaramella's theories, that Russian agents illegally brought uranium into Italy.
Neither his lawyer nor prosecutors were reachable for comment yesterday.Reuse content