Sergei Skripal: Who, and what, could be behind 'poison' attack?

A former Russian spy and his daughter remain in hospital in comas after being poisoned in Salisbury.Mystery surrounds how they came into contact with a nerve agent and who could have administered it, as well as what their motive would have been.Sergei Skripal and his daughter have been hit with a nerve agent, which interrupts the messaging between the brain and the body.