The death of former Russian spy Alexander Litvinenko by radiation poisoning has raised the issue of who else might have been contaminated.
Alexander Litvinenko died in hospital from radiation poisoning
Mr Litvinenko, an ex-KGB officer and critic of Russian President Vladimir Putin, died last week of radiation poisoning attributed to polonium-210.
BBC security correspondent Frank Gardner answers questions surrounding the former spy's death.
Could it have been suicide?
Let's straight away rule out that this could have been suicide on Mr Litvinenko's part. The police have to keep an open mind on this, they have to look at all possibilities.
They haven't totally ruled that out publicly, but privately they have, because if you're going to kill yourself, going this way is not really a pleasant way to go. It's slow and agonisingly painful.
Will Mario Scaramella, a contact of Mr Litvinenko, testing positive for polonium-210 help the investigation?
It will do. It's pretty grim news for Mr Scaramella whom I suspect is probably in shock.
What it probably help the police with is in terms of constructing a timeline. They're putting together a lot of CCTV footage, testaments from witnesses, various other bits of evidence and clues.
They're trying to establish exactly when Mr Litvinenko was poisoned and where. Was it on a flight from Moscow? Was it at Itsu? They're trying to work all these things out.
What is significant about the use of polonium-210 in this case?
Before all of this hardly anyone had heard of polonium-210. The only reason I'd heard of it was because I'd read it in a Frederick Forsyth novel.
Now we're learning more about it. The fact that the top toxicologists in London at the University College Hospital couldn't identify for many weeks what was wrong with Mr Litvinenko shows how unusual and how rare this element is.
What people aren't certain about is how long it takes to kill you and in what doses.
Is there still Russian spying activity going on in Britain, even though the Cold War has ended?
Security sources in Whitehall have told the BBC Russian intelligence officers spying in Britain are every bit as active today as they were during the Cold War.
They say there are more than 30 known Russian spies operating in Britain, for both their overseas intelligence service, the SVR, and Russia's military intelligence arm, the GRU.
They are believed to report directly to two controllers in London, known as "the Residents".
Some of these agents are said to be using diplomatic cover, others are working undercover as journalists.
As well as monitoring prominent Russian dissidents, their aims include trying to acquire military, technological, economic and political secrets through covert means.
British security sources said the Russian spies are still using many of the traditional tools of espionage such as dead letter drops and attempts to recruit new sources at public events like trade fairs, but that they had also updated their methods with new technology.
Britain's security service, MI5, is fully engaged in trying to counter their efforts but today, with counter-terrorism taking up the lion's share of their time, only six per cent of MI5's resources go into counter-espionage, compared to well over 50% during the Cold War years.
Security chiefs admit that leaves a significant risk from Russian espionage.