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, the 43-year-old 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 center does so without any regard for human life,” Horwell said, repeating an accusation by the lawyer representing Litvinenko's widow, Marina, that “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.
2 Russian suspects
Horwell said police wanted two Russians – Dmitry Kovtun and Andrei Lugovoy, a former Russian agent who since 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.
Russian businessman Dmitry Kovtun denies role in Litvinenko’s death and, during news conference in Moscow on April 8, 2015, offers to give evidence at inquiry.
The inquiry has been told that traces of polonium were found across London where the two men had been: in offices, hotels, planes and even Arsenal football club's stadium.
"It is the scientific evidence that condemns Lugovoy and Kovtun," Horwell said.
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 year's end.