The nerve agent that poisoned a couple near Salisbury was unlikely to have been left in the open before they touched it, a government scientist has told BBC News.
The Novichok was so toxic it was able to pass through the skin and did not need to be ingested, he added.
Police have said the couple from Amesbury were exposed to the substance after handling a contaminated item.
Charlie Rowley, 45, and Dawn Sturgess, 44, remain critically ill.
Speaking to BBC security correspondent Frank Gardner, the source said Novichok could be degraded by rainwater and sunlight over time – meaning it was probably discovered by the couple in a contained space.
The source added that Mr Rowley and Ms Sturgess’s symptoms were the same as those shown by Russian ex-spy Sergei Skripal and his daughter Yulia.
They were both poisoned with Novichok in nearby Salisbury in March.
Who are the victims?
Ms Sturgess is understood to be a mother of three with links to John Baker House, which offers supported accommodation. It has been cordoned off by police.
A close friend of Ms Sturgess’s, who lived in the same building, described her as a “loving and caring person” and said – contrary to previous claims – she “never did drugs”.
Charlie Rowley’s brother Mark told the BBC: “He’s a lovely guy and would do anything for you. He’s a sweetheart basically.”
What happened to them?
On Saturday, paramedics were called twice to a flat in Muggleton Road in Amesbury – first at 11:00 BST after Ms Sturgess collapsed.
Medics attended again several hours later, after Mr Rowley also fell ill.
A friend of the couple, Sam Hobson, said after Ms Sturgess was taken to hospital, he and Mr Rowley went to a chemist in Amesbury to collect a prescription before going to an event at a nearby Baptist church.
The two men returned to the flat and planned to visit the hospital but Mr Rowley “started feeling really hot and sweaty” and began “acting all funny”, Mr Hobson, 29, said.
“He was rocking against the wall and his eyes were red, pinpricked, and he started sweating loads and dribbling, so I had to phone an ambulance for him,” said Mr Hobson.
Wiltshire Police initially thought the pair had fallen ill after using a contaminated batch of heroin or crack cocaine.
But after tests at the government’s military research facility at Porton Down, a major incident was declared and it was confirmed the couple had been exposed to Novichok.
What is Novichok?
Novichok, which means “newcomer” in Russian, is a group of advanced nerve agents developed by the Soviet Union in the 1970s and 1980s.
The nerve agent can come in liquid form, but is also thought to exist as a solid which could be dispersed as an ultra-fine powder.
Some of the agents are reported to be “binary weapons”, which means the agent is stored as two less toxic chemicals that are easier to transport, handle and store.
The nerve agent blocks messages from the nerves to the muscles, resulting in convulsions, interrupted breathing, vomiting and, in most cases, death.
In March Russian ex-spy Sergei Skripal and his daughter Yulia were found collapsed in Salisbury town centre.
They had been poisoned by Novichok, after coming into contact with the toxic substance at the front door of their home.
Yulia spent more than a month in hospital before being discharged, while Sergei spent more than two months.
The British government said Mr Skripal, a former double agent imprisoned in Russia, was targeted and accused the Russian state of involvement. Russia denies the accusation.
The pair are now in a secure location.
What are the police investigating?
Police do not believe Mr Rowley and Ms Sturgess were the victims of a targeted attack, but instead came into contact with the substance somewhere in Amesbury or Salisbury.
Home Secretary Sajid Javid said the “strong working assumption” was that the couple came into contact with Novichok in a location which had not been cleaned up following the Skripal poisoning.
Assistant Commissioner of Specialist Operations Neil Basu said that “around 100 detectives” from the Counter Terrorism Policing Network (CTPN) were working on the investigation.
Five areas have been cordoned off: Muggleton Road, Boots pharmacy and the Baptist church in Ambesbury; John Baker House and Queen Elizabeth Gardens in Salisbury.
What has the government said?
Mr Javid called on Russia to explain “exactly what has gone on”, adding: “We will stand up to the actions that threaten our security.”
The home secretary said he was “comfortable” the “exact same nerve agent” had been used in both the Salisbury and Amesbury poisonings – but added it was not yet known if they were from the same batch.
In response to Mr Javid’s comments, Russia said the British government was subjecting them “to hell”.
Russian foreign ministry spokeswoman Maria Zakharova urged police not to be led by the “dirty political game” and said she was confident London would have to apologise to Russia.