The British envoy in Russia says that he expects the probe into the poisoning of former Russian agent Alexander Litvinenko to end within weeks.
Mr Litvinenko was a known critic of Russia's security services
Anthony Brenton told the BBC the UK government would push for any Russians charged over the case to be extradited.
Meanwhile, one of the chief suspects, Andrei Lugovoi, told a Moscow radio station he was a witness not a suspect.
Former KGB agent Mr Litvinenko died in November in London after being poisoned by radioactive substance polonium-210.
Mr Lugovoi, who is also formerly of the KGB, and two other Russians met Mr Litvinenko on 1 November, the day he fell ill.
Traces of polonium-210 have been found in a several places Mr Lugovoi visited in London.
"Our position is that we want to catch the person who committed this crime and see them punished," Mr Brenton told the BBC.
Mr Lugovoi has denied being involved in the poisoning
"We will do everything that we need to do to achieve that result. And if that involves extraditing someone from Russia then we will try to achieve that."
But, says the BBC's Richard Galpin in Moscow, it could be a long battle. Russian prosecutors have already said they will reject any extradition request.
Mr Lugovoi, meanwhile, told the radio station that he planned to ask British officials for more information about the progress of the case.
He also expressed concern that media coverage of the case in the UK meant that he would not be fairly treated.
"If you conducted a survey, 99% would say that we carried out this killing," he said. "So it's necessary to determine: Is it worth meeting with people who from the start are inclined to treat us unfairly?"
Mr Litvinenko, a critic of Russian President Vladimir Putin and the Russian security services, was granted asylum in the UK in 2000.