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British police suspect Russians struck former Russian double agent - sources

WASHINGTON/LONDON (Reuters) - Britain's main line of inquiry is that Russians used a mystery substance to strike down former Russian double agent Sergei Skripal, two security sources said.

Skripal, who betrayed dozens of Russian agents to British intelligence, and his 33-year-old daughter, Yulia, were found slumped unconscious on a bench outside a shopping centre in the southern English city of Salisbury on Sunday afternoon.

British investigators believe Russians may have used the mystery substance to harm Skripal in revenge for his treachery, a U.S. security source who spoke on condition of anonymity said.

A European security source, who also spoke on condition of anonymity, confirmed that this was the main line of inquiry. The sources did not give details about the mystery substance.

(Reporting by Mark Hosenball and Guy Faulconbridge; editing by Michael Holden)