Last week, it was revealed that investigators had identified, from CCTV footage, Russian nationals that were suspects in the poisoning of the Skripals.
"They [the investigators] are sure they [the suspects] are Russian", said the source, adding security camera images had been cross checked with records of people who entered the country.
A source who has been briefed on the progress of the investigation said: "They want to get out of Charlie the best account of where he was and what happened so they can get to the bottom of this matter".
Detectives are investigating the possibility - described by Mr Rowley's brother - that Ms Sturgess tested the spray on her face and hands, believing it to be perfume. "Obviously he's stronger than I thought".
The New York Times previously reported that British investigators believed the father and daughter were likely poisoned by current or former agents of Russia's military intelligence service, known as the GRU. He was freed in a spy swap in 2010 and settled in the UK.
Britain blamed Russian Federation for the poisoning of the Skripals.
Mr Rowley and the Skripals survived, but Ms Sturgess died on 8 July.
It is unclear where Rowley found the bottle, or whether it contained the same batch of nerve agent used on the Skripals.More news: Google hit with record 4.3 billion-euro fine
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Novichok victim Charlie Rowley has been released from hospital.
'We don't know yet if he had direct contact with the nerve agent like Dawn appears to have done or whether it was after he had touched her.
Paul Cosford of Public Health England said it was safe for Rowley to leave hospital.
Mr Rowley's brother Matthew has said his sibling picked up a perfume bottle containing the toxic substance.
The Organisation for the Prohibition of Chemical Weapons (OPCW) will analyse the substance before reporting back its conclusions.
An inquest into the death of Sturgess opened on Thursday in Salisbury and was adjourned until a pre-inquest review on January 16, pending further investigation. They set out to determine the place, time and type of death, but do not apportion blame.
Emergency workers in protective suits search around John Baker House Sanctuary Supported Living after a major incident was declared when a man and woman were exposed to the Novichok nerve agent on July 6, 2018 in Salisbury, England.