Investigators scour Wiltshire to locate Novichok item
Officers are trawling through more than 1300 hours of CCTV to find contaminated item.
Forensic investigators will continue to comb for clues in Wiltshire after the latest Novichok poisonings - as police are expected to remain in the county for weeks to come.
Salisbury and nearby Amesbury are being scoured for traces of the potentially deadly nerve agent after Dawn Sturgess, 44, and her partner, Charlie Rowley, 45, were taken ill last Saturday.
Both fell ill after "handling a contaminated item" and remain in a critical condition, Scotland Yard said.
Investigators wearing camouflage protective clothing entered the John Baker House assisted-living accommodation in Salisbury, where mother-of-three Ms Sturgess lives, after they took a sample from the outside of the building on Friday.
Other sites visited by the couple in the lead-up to their hospitalisation are also being looked at, as detectives piece together a timeline of the couple's movements.
Officers have spoken to several key witnesses, and are trawling through more than 1,300 hours of CCTV which has been collected so far.
Those in camouflage at John Baker House were followed in by two others, one of whom was taking pictures, as they began combing for the deadly substance.
They took a swab from the exterior of the building on Rollestone Street, which is now under a heavy cordon and lined by forensic tents.
There was also a heavy operational presence at Mr Rowley's flat, where they were both taken ill.
Incident response vehicles and fire engines joined police at his Amesbury home.
Police have been unable to locate the source of the contamination and have not ruled out more people falling ill from coming into contact with the substance left over after Sergei and Yulia Skripal were targeted.
One theory understood to be under investigation is whether the pair inadvertently found the container used to transport the nerve agent in the Skripal attack before being recklessly discarded.
Mr Rowley has been described as having foraged for goods to fix and sell, and is known to have collected discarded cigarettes.
The Metropolitan Police said: "Due to the unique challenges involved with this operation, police activity is expected to take weeks and months to complete.
"The focus of the investigation remains identifying the source of the contamination as quickly as possible. "
The second nerve agent emergency in four months prompted a diplomatic row, with Home Secretary Sajid Javid accusing the Russian state of using Britain as a "dumping ground for poison".
The Russian Embassy hit back, accusing the Government of trying to "muddy the waters" and "frighten its own citizens".
The Defence Science and Technology Laboratory at Porton Down confirmed on Wednesday that the victims had been exposed to Novichok.
Novichok remains highly toxic for a considerable period of time, so even the tiniest trace remaining in a container picked up by the victims could account for their severe illness.
The episode in Salisbury - the first use of a nerve agent in Europe since the Second World War - sparked international outrage.