A HERO policeman who became seriously ill as he tried to help Sergei Skripal and his daughter Yulia was poisoned at the spy's home as he hunted for evidence, senior officers believe.

Detective Sergeant Nick Bailey, who lives in Alderholt, had been searching for drug paraphernalia or a suicide note at Mr Skripal's property in Salisbury when he was poisoned with a nerve agent.

Wiltshire Police's Chief Constable Kier Pritchard said he had watched evidence from body-worn cameras used by officers who first attended the scene on March 4. Their response to the incident was “first class”, he said.

“We wouldn't have known from those first hours what we were dealing with. At that time we didn’t know if there was anything other than a medical incident, something that was drug-related or something more sinister,” he said.

Chief Constable Pritchard said officers were looking for information to establish a timeline of events and explain why the Skripals had fallen “gravely ill”.

“That [information] could be a suicide note, it could be evidence of drugs, it could be evidence of some form of substance,” he said.

Wiltshire Police has established a 'welfare cell' to help affected officers with the psychological impact of the poisoning.

“This has been a long three months for many of us," Chief Constable Pritchard said.

"My heart absolutely goes out to Nick and his family over all that they’ve suffered.”

As reported in the Daily Echo, military personnel and police spent a day in Alderholt in March as part of their investigation. A car belonging to DS Bailey was recovered.

Mr Skripal and Ms Skripal were found collapsed near a shopping centre in Salisbury after a nerve agent attack.