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Media captionThe victims were “targeted specifically” – Assistant Commissioner Mark Rowley

A nerve agent was used to try to murder a former Russian spy and his daughter, police have said.

Sergei and Yulia Skripal were found unconscious in Salisbury on Sunday afternoon and remain critically ill.

A police officer who was the first to attend the scene are in a serious condition in hospital, Assistant Commissioner Mark Rowley said.

Nerve agents are highly toxic chemicals that stop the nervous system working and shut down bodily functions.

They usually enter the body through the mouth or nose, but can also be absorbed through the eyes or skin.

Russian spy: What we know so far The ‘quintessentially English' home of a Russian spy Sergei Skripal and the 14 deaths under scrutiny What now for UK-Russia relations ? Media captionCCTV has emerged of Sergei Skripal at a store in Salisbury simply five days before he and his daughter collapsed Image caption Mr Skripal, 66, and his daughter Yulia, 33, collapsed on a bench in Salisbury city centre Image caption Col Skripal, 66, had been living in Salisbury after being released by Russia in 2010 Read more about Sergei Skripal's background here . Putin, power and poison: Russia‚Äôs elite FSB spy club