MICHELE NORRIS, host:

The former Russian spy who claims he was poisoned has died. Alexander Litvinenko was a fierce critic of Russian President Vladimir Putin and his government. He had been investigating the killing of Russian journalist Anna Politkovskiya, also a critic of the Russian government. She was gunned down at her Moscow apartment last month. Litvinenko said he became sick after meeting two Russians in a hotel back on November 1. Although he claims he was poisoned, an official cause of death has not yet been determined.

Joining now from London is NPR's Rob Gifford, who's been following this story. And Rob, give us the latest on what's happened today.

ROB GIFFORD: Well, yes, as you say, the latest is that the hospital has confirmed that Alexander Litvinenko died at 9:21 local time this evening, that's 4:21 East Coast time in the United States. His condition had actually deteriorated rather badly overnight last night. He was put on life support and it became clear later in the day that he really wasn't going to survive the day.

The police have also issued a statement saying it's an unexplained death, just as the hospital, as you mentioned, have said they really don't know what it was that killed him.

NORRIS: There has been such a medical mystery, it seems. Help us understand sort of the twists and turns on this. The doctors haven't determined yet a cause of death, but what do they suspect?

GIFFORD: Well that's right. It is a mystery. Earlier this week, the toxicologist treating Mr. Litvinenko, who is one of the most senior experts in the country, and he said he was sure that it was Thalium, this heavy metal, a pinch of which in somebody's food can actually kill them.

And then by about Tuesday, they were saying they weren't sure. They were looking at possibilities of radiation. Now today during the day, they've said again they don't think it was radiation or Thalium. And it really is a mystery as to what exactly has killed Mr. Litvinenko. And of course there will now be a post mortem and perhaps they will be able to ascertain exactly what it was.

NORRIS: And what's been the reaction from Moscow on this story?

GIFFORD: Well, the Kremlin all along has denied any kind of involvement in the alleged poisoning of Alexander Litvinenko. They have issued rigorous denials, going so far as to say that Alexander Litvinenko is not the kind of person over which they would be willing to sour ties with the United Kingdom. So, they've been absolutely adamant that they haven't been involved.

But equally on the other side, Litvinenko's friends here in the London - and there's a large dissident community here from Russia - have said absolutely, it was the Kremlin that was involved. It was clear that Litvinenko was an enemy of the Kremlin. He was very vociferous in his criticism of Vladimir Putin and his policies. And they maintain that it was Russian government involvement that killed Mr. Litvinenko finally today.

NORRIS: Rob, what's likely to happen now as far as the dissident community in London is concerned? They must be worried right now.

GIFFORD: I'm sure they are. And very interestingly, something has come up just in the last couple of days. On that day when Litvinenko says he was poisoned by the two Russians who he met early in the day, later in that day he met an Italian security expert called Mario Scarimella(ph), and Scarimella on Tuesday said that he had received an e-mail that was naming the killers of Anna Politkovskiya, the journalist who was killed in Russia in October. And he had gone to see Litvinenko on that day to warn him that Scarimella himself, this Italian security expert, and Litvinenko were also on the hit list.

As it turned out, Litvinenko had already met with the Russians who he alleged before he died had poisoned him. So there is clearly a web of both dissidents and possibly agents out there, and I think certainly the dissidents, if Mr. Scarimella is right in his accusations, are going to be very concerned that what has happened to Mr. Litvinenko could possibly happen to them.

NORRIS: That was NPR's Rob Gifford speaking to us from London. Thank you, Rob.

GIFFORD: Thanks very much, Michele. Transcript provided by NPR, Copyright NPR.