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 is now 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 normally enter the body through the mouth or nose, but can also be absorbed through the eyes or skin.
Rowley, head of Counter Terrorism Policing, said government scientists had identified the agent used, but would not make that information public at this stage.
“This is being treated as a major incident involving attempted murder, by administration of a nerve agent,” he said.
“Having established that a nerve agent is the cause of the symptoms… I can also confirm that we believe that the two people who became unwell were targeted specifically.”
He said there was no evidence of a widespread health risk to the public.
Two other police officers who attended the scene were treated in hospital for minor symptoms, before they were given the all clear.
It is understood their symptoms included itchy eyes and wheezing./BBC