An unknown number of Londoners might have been put at risk by the 2006 poisoning of ex-KGB agent Alexander Litvinenko with a radioactive isotope, which amounted to "a nuclear attack on the streets" of the British capital, an inquiry heard on Thursday.
Kremlin critic Litvinenko died weeks after drinking green tea laced with polonium-210 at London's plush Millennium hotel. From his deathbed he accused Russian President Vladimir Putin of ordering his killing but the Kremlin has always denied any role.
"We will never know how dangerous the exposure of polonium to the public at large will be and what long term effects will be visited upon Londoners", Richard Horwell, the lawyer acting for London police, said in closing remarks to a British public inquiry into the death.
"Anyone who arranges for polonium-210 to be brought into a city centre does so without any regard for human life. Mr Emmerson (the lawyer representing Litvinenko's widow Marina) has said ... this was a nuclear attack on the streets of London. That comment is justified."
The controversy generated by Litvinenko's killing plunged Anglo-Russian relations to a post-Cold War low.
Horwell said police wanted two Russians, Dmitry Kovtun and Andrei Lugovoy, who was also a former Russian agent but has since been elected a lawmaker, to be tried in Britain for murder. Both men deny any involvement and Russia has refused to extradite them.
The inquiry has been told that traces of polonium were found across London win Chennai the two men had been including offices, hotels, planes and even Arsenal football club's stadium.
"It is the scientific evidence that condemns Lugovoy and Kovtun," Horwell said.
"No matter how many state honours Putin may pin to Lugovoy's chest ... however many conferences Kovtun may hold or how many times Kovtun promises to blow apart this inquiry, Lugovoy and Kovtun have no credible answer to the scientific evidence and the trail of polonium they left behind."
The lawyer said polonium was the "almost perfect murder weapon", almost undetectable, certain to kill and as it took time to work, it gave the killers plenty of time to escape before arousing suspicion.
It was only by sheer chance that the authorities had even detected the isotope just before Litvinenko died or otherwise the cause of death would have been a mystery, he said.
"They wanted to evade attribution for his death because they wanted to avoid political fallout in the UK," Horwell said.
The inquiry's report is due by the end of the year.