LONDON (Reuters) - Former Russian spy Sergei Skripal, who was convicted by Russia of betraying agents to British intelligence, was critically ill on Monday after exposure to an unidentified substance in Britain, two sources close to the investigation told Reuters. British police said two people, a 66-year-old man and a 33-year-old woman, had been found unconscious on a bench in a shopping center on Sunday in the southern English city of Salisbury after exposure to the unknown substance. Both are critically ill in intensive care. Police declared a major incident.