Britain now says the Russian spy, his daughter, and the police officer who are in hospital were poisoned by a nerve agent. They are looking for the source of the nerve agent.
Counter-terrorism officers are working to uncover the origin of the nerve agent used in the attempted murder of ex-Russian spy Sergei Skripal and his daughter, Yulia, in Salisbury.
The pair were found unconscious and are critically ill in hospital, along with a police officer who went to their aid.
A source told BBC News the chemical used on Sunday was likely to be rarer than Sarin or VX nerve agents.
(more)
The Home Secretary is expected to make a statement on the developments.