Spy may have been poisoned through air vents in BMW
The nerve agent used in the attempted assassination of Sergei Skripal and his daughter was probably delivered though his car's ventilation system.
Sources have said that intelligence officials "now have a clearer picture of just how the attack was conducted".
According to US reports, security agencies now believe the toxin - identified as a fourth generation nerve agent called Novichok - was used in a "dust-like powdered form" and that it circulated through the vents of Mr Skripal's BMW.
Citing three intelligence officials, ABC News said the Novichok agent was likely to have been delivered in a powdered form.
"It is a Cold War substance, something they [Russia] claimed never to have," an intelligence official told the US network.
A chemical weapons expert has said it would need just 5 microns, or 0.005 millimetres, of the agent placed in the car's air vents or air con system for it to be effective.
The poison would have blown around the car in an extremely fine cloud the Skripals would not have noticed when they turned on the heating system, during what had been a very cold spell.
It is being suggested that the reports that Mr Skripal was seen behaving in an angry manner, shouting and "acting incoherently" while eating with his daughter Yulia in Zizzi's restaurant in Salisbury shortly before they both collapsed are consistent with the early stages of exposure to a nerve agent.
Local sources say the father and daughter are "close to death" and it is thought "highly unlikely" they will recover.
Jerry Smith, a security expert who used to work for the Organisation for the Prohibition of Chemical Weapons in The Hague, said: "The theory that the poison could have come through the car's air vents in a micro-fine powder is perfectly plausible in my experience."
A spokesman for Scotland Yard, whose counter-terrorism unit is leading the investigation into the attack, said: "We are still trying to determine how the nerve agent was applied and are looking at numerous avenues as part of our inquiry."
Detectives are also investigating the possibility the nerve agent may have been inadvertently brought from Russia by Yulia in her luggage. The timing of the attack a day after Yulia arrived in Britain is not thought to be a coincidence.