MARY LOUISE KELLY, HOST:

There will be consequences - that warning today from the British government after the apparent attempted murder of a former Russian spy and his daughter. U.K. Home Secretary Amber Rudd delivered the warning this morning in London at the House of Commons.

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AMBER RUDD: The use of a nerve agent on U.K. soil is a brazen and reckless act. This was attempted murder in the most cruel and public way.

KELLY: The ex-spy is named Sergei Skripal. He's a former colonel in Russian military intelligence. He and his daughter are still in the hospital, still in critical condition. Police believe they were poisoned by a nerve agent. At least 19 others in the area also suffered some exposure. Luke Harding is a reporter for The Guardian in London and the author of a book on another ex-Russian spy poisoned on British soil, Alexander Litvinenko. Harding says this attack was meant to send a message.

LUKE HARDING: The fact that nerve agent was used, which inevitably would be discovered, is a calling card. And, of course, Moscow is formally denying any involvement - saying the British are being hysterical and jumping to conclusions and being Russophobic and so on. But I think inevitably the government will be thinking that this is yet another operation by Vladimir Putin's spy services.

KELLY: When you say that this appears to be some sort of calling card for Russia, why? How so?

HARDING: Because there are numerous ways of killing people - but the thing about a nerve agent is that it's a kind of state thing. It's kept traditionally in military stockpiles. And the experts we've spoken to have said it's not something you can build from an instruction manual on the internet. It actually requires technical skill. It requires research and development capacity. And, of course, the other thing to note is that the Soviet Union famously had a poisons factory. There's a long tradition of defectors, of enemies, of traitors and (unintelligible) being snuffed out by the KGB. We've actually seen a revival of these techniques under Vladimir Putin over the last 18 years, with a series of critics dying in very mysterious and often ghastly circumstances.

KELLY: So that is the means that was apparently used to carry this out. What about motive? Skripal - we should say - he was a Russian spy. He was convicted of selling secrets to Britain, to MI6. But he was pardoned. He left Russia 10 years ago in this dramatic spy swap between the U.S. and Russia. Why would someone go after him now?

HARDING: Well, I mean, I think if you were to decode it, it's a chilling message to anybody who is thinking about cooperating with the CIA or with MI6 or with other kind of Western sort of special services - that we can strike at you whenever we like. I mean, Skripal had exited from Moscow in this spy swap in 2010. He had grown accustomed to life in England. He went to the pub. He bought lottery scratch cards. I think it's fair to say he had let his guard down. He thought the ghosts from the past would not catch up with him. And essentially, the message from Moscow - if it is Moscow, and I think it probably is - is we are the masters in this situation in this game of espionage. And we can do whatever we want, where we want and when we want.

KELLY: If this was Russia's hand at work, what kind of pressure is Britain under to respond?

HARDING: It's very hard for Britain to articulate a response. One thing Boris Johnson, the foreign secretary, said yesterday there would be no U.K. representation at this summer's soccer World Cup, which is being hosted by Russia, of course - no Prince William.

KELLY: It's not exactly a forceful response to an assassination on British soil.

HARDING: Yeah, it's not really going to strike terror...

KELLY: No.

HARDING: ...into the hearts of the KGB - strike - FSB. But it's hard. I mean, the question is how do you confront a rogue or semi-rogue power that is prepared to send out killers to rub out people it doesn't like abroad. And at the moment, the government of Theresa May does not really have an answer.

KELLY: Luke Harding, he's a reporter for The Guardian newspaper - also author of "A Very Expensive Poison." Thanks, Luke.

HARDING: Thank you. Transcript provided by NPR, Copyright NPR.