A handout image released Nov. 20 2006, shows Alexander Litvinenko, at University College Hospital in London

British police say they are now treating the investigation into the death of former Russian spy Alexander Litvinenko as a murder case.

The London police statement came as British authorities also announced discovery of radiation traces at their country's embassy in Moscow, but stressed that the amount posed no threat to health.

Meanwhile, British detectives probing Litvinenko's radiation poisoning death witnessed the interrogation by Russian investigators of businessman Dmitri Kovtun, one of two Russians who met with the victim November 1, the day he fell ill. Litvinenko died three weeks later in London of acute radiation poisoning. Toxicologists found high levels of the radioactive isotope polonium-210 in his body.

A flag over the British embassy in Moscow

The detectives also plan to witness an interview with former spy Andrei Lugovoi, the second Russian currently being examined at a Moscow hospital for possible radiation contamination.

Russian news reports say the Britons are interested in Lugovoi because planes on which he flew to and from London and his hotel rooms in the British capital were found to be contaminated with radiation.

Meanwhile, another witness who met with Litvinenko November 1, Mario Scaramella, an expert on Soviet-era KGB activities, has been discharged from a London hospital that had been monitoring him for radiation contamination. Scaramella, who identifies himself as an ally of Litvinenko, was found contaminated with the same radiation as the late former spy but in much lower, non-life threatening doses.

Some information for this report was provided by AFP, AP and Reuters.