The origins of the nerve agent used in the attempted murder of former spy Sergei Skripal and his daughter Yulia are now the major focus for British counter-terrorism investigators.
Scientists at Porton Down military base have identified the nerve agent but are refusing to disclose what it is.
Speculation is mounting that the attack could have only been carried out by a state, with suspicion falling on Russia.
Former UN weapons inspector Jerry Smith told Euronews that only a well-funded, well-organised, well-equipped organisation could have executed the poisoning.
"Getting hold of the material, getting hold of the facility to actually produce this, particularly as you'd have to do this covertly would be a real challenge. The number of countries or organisations that potentially could produce that is rather large. If we get into some of the more high tech, more modern agents, then we start to narrow down the sort of capabilities and countries therefore that could be able to produce that."
The nerve agent used in Salisbury is not thought to be VX. Those close to the investigation are reported to have said it is rarer than Sarin gas.
For the moment, it is still unclear how the poison was administered, but Smith says the most likely way was probably via the skin.
"At this stage I think what we can say is that if the police office was contaminated by the same material that has affected the victims, that would suggest that perhaps the material passed from them onto him whilst he was responding to them which would then perhaps lead to something on their skin or on their clothing rather than something that was actually ingested inside them there."
Wiltshire police say an officer, who was in intensive care after going to help Skripal and his daughter, is now "stable and conscious".
While Skripal and his daughter are still fighting for their lives in hospital, Smith says they could still survive.
"Conceivably this is recoverable depending on how quickly they were treated for actual nerve agent poisoning. There are also ways that recovery can be aided and speeded up."