Police in hazmat suits have been searching a Salisbury hostel as they try to find the item contaminated with a nerve agent that poisoned a couple.
Investigators entered John Baker House, where one of the victims, Dawn Sturgess, 44, lives, on Friday afternoon.
Police believe Charlie Rowley, 45, and Ms Sturgess were exposed to Novichok after handling the unknown object.
The pair remain in a critical condition in hospital.
Police said the "unique challenges" surrounding the operation meant "police activity is expected to take weeks and months to complete".
They added they are looking through more than 1,300 hours of CCTV footage to help them find the source of the contamination.
BBC home affairs correspondent Daniel Sandford said no objects have been collected for testing yet.
A government scientist told BBC News that the contaminated item was unlikely to have been left in the open before the couple touched it.
Novichok can be degraded by rainwater and sunlight over time - meaning it was probably discovered by the pair in a contained space, the government source added.
But other experts have said the nerve agent was designed to be persistent and not break down.
Speaking to BBC security correspondent Frank Gardner, the government scientist also said the Novichok was so toxic it was able to pass through the skin and did not need to be ingested.
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.
Extensive and painstaking search
By Frank Gardner, BBC security correspondent
The Amesbury investigation is entering a new phase.
Scientists and chemical weapons experts at Porton Down in Wiltshire are bracing themselves for an influx of countless objects all needing forensic analysis.
Investigators, led by the Met Police's counter-terrorism detectives, are working on the basis that the pair - now critically ill from Novichok poisoning - handled a contaminated item that caught their interest.
They need to find it.
The item - or possibly items - are thought to be something found and touched by the pair, possibly something as innocuous as a perfume bottle or other luxury toiletry.
The search for this "source item" is likely to be extensive, painstaking and could last through the summer.
Who are the victims?
Ms Sturgess is understood to be a mother of three who lives at the Salisbury hostel, which offers supported accommodation.
A close friend of Ms Sturgess's, who also lived at John Baker House, described her as a "loving and caring person".
Mr Rowley's brother Matthew 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.
Based on information from a friend and items found at the flat, 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 they had been exposed to Novichok.
What are the police investigating?
Police do not believe Mr Rowley and Ms Sturgess were victims of a targeted attack, but instead came into contact with the substance in Amesbury or Salisbury.
On Friday night, police revealed more details around the victim's movements.
They said the pair were both at John Baker House on Friday lunchtime, before visiting a number of shops in Salisbury and heading to Queen Elizabeth Gardens.
They returned to the hostel at around 16:20 BST before heading to Amesbury by bus at around 22:30.
Police believe they were there until emergency services were called on Saturday.
Five areas have been cordoned off: Muggleton Road, Boots pharmacy and the Baptist church in Amesbury; John Baker House and Queen Elizabeth Gardens in Salisbury.
Home Secretary Sajid Javid said the "strong working assumption" was that the pair 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 were working on the investigation.
Anger at Russia
Conservative MP Tom Tugendhat, chair of the Foreign Affairs Committee, said the latest incident was a result of a "war crime" by Russia - and that using a nerve agent in a civilian area was a "vile act of terror".
Mr Javid called on Russia to explain "exactly what has gone on".
The home secretary added 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.
But Russia - which denies involvement in either incident - said the UK was trying to "muddy the waters" and "intimidate its own citizens".
UK reaction to Amesbury incident puzzling. Russia proposed a joint investigation into Salisbury from the very outset, proposal remains valid. Instead of cooperating, London prefers to muddy the waters, confuse and intimidate its own citizens.— Russian Embassy, UK (@RussianEmbassy) July 5, 2018
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 the UK would have to apologise to Russia.