Investigators find the highest concentration of the nerve agent Novichok on the Skripals’ front door.
British police and intelligence agencies have identified key suspects in the attempted assassination of Sergei Skripal and his daughter Yulia, the Daily Telegraph reports.
Counter-terrorism police are building a case against "persons of interest" believed now to be in Russia.
The Telegraph has been told the criminal inquiry, expected to take many more months, has made a breakthrough in identifying key people over the nerve agent attack.
It is thought that a search of flight manifests in and out of the UK has yielded specific names in the hunt for the Skripals' would-be assassins.
Police have also drawn on CCTV footage in Salisbury and trawled car number plate recognition cameras.
However, counter-terrorism police will hit a diplomatic brick wall in trying to interview — let alone prosecute — the suspects.
Investigators have privately admitted the huge difficulties in bringing charges, while any demands made by British authorities for access to the "persons of interest" will escalate tensions between the UK and Russia.
Authorities believe Colonel Skripal, 60, a Russian double agent who spied for Britain, was targeted at his home in Salisbury by a Kremlin-backed hit squad which smeared the novichok nerve agent, in liquid form, on his front door.
The emails of Yulia Skripal, 33, who lives in Moscow, were monitored prior to her flight to the UK to visit her father, giving the hit squad notice of when he would be at home.
Authorities were foiled in the prosecution of the prime suspects in the murder of Alexander Litvinenko, the former Russian agent who was poisoned with the radioactive substance polonium-210 at a London hotel in 2006.
Police identified Andrey Lugovoy and Dmitry Kovtun — based on Litvinenko's account before he died and forensic evidence of a radiation trail — but Russia refused the UK's extradition requests. Both men deny wrongdoing and Lugovoy was given further protection after being made a Russian MP.
Yulia Skripal. Photo / Facebook
Alexander Yakovenko, Russia's ambassador to the UK, yesterday claimed British authorities had injected the Skripals with nerve agent developed at the military research laboratory 16km from Salisbury.
Russia has demanded access to Skripal, who has been in hospital since the March 4 attack but is no longer in life-threatening condition, and to Yulia Skripal, who was discharged a week ago.
The decontamination of Salisbury began in earnest yesterday with police cordons in nine "hot spots" where it is feared traces of the nerve agent are lingering. Officials insisted the city is safe for residents and visitors.
