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.