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FILE - In this Wednesday, Aug. 9, 2006 file photo, Sergei Skripal speaks to his lawyer from behind bars seen on a screen of a monitor outside a courtroom in Moscow. It has been reported on Monday, March 5, 2018 by the British media that Skripal is in critical condition after exposure to 'unknown substance' in English city of Salisbury. (AP Photo/Misha Japaridze, File)
FILE - In this Wednesday, Aug. 9, 2006 file photo, Sergei Skripal speaks to his lawyer from behind bars seen on a screen of a monitor outside a courtroom in Moscow. It has been reported on Monday, March 5, 2018 by the British media that Skripal is in critical condition after exposure to 'unknown substance' in English city of Salisbury. (AP Photo/Misha Japaridze, File)

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Former Russian spy Sergei Skripal has been left critically ill after being exposed to an 'unknown substance'. CNN's Nick Paton Walsh takes a look at the man behind the headlines.

Who is Sergei Skripal?

Former Russian spy Sergei Skripal has been left critically ill after being exposed to an 'unknown substance'. CNN's Nick Paton Walsh takes a look at the man behind the headlines.