They had gone to Queen Elizabeth Gardens in Salisbury on the previous evening, where it is thought Ms Sturgess could have picked up a glass bottle containing the Novichok from the banks of the River Avon.
In a statement last week, Scotland Yard said the nerve agent was found in a "small bottle" at Charlie's home, but did not elaborate.
Sturgess died in the hospital about a week later.
Mr Rowley is now said to be under police guard after he was discharged from hospital following a 20-day stay, much of which was spent...
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.
"I'm very surprised because when I first saw him I thought he would be there for months", he said.
Britain's Press Association, citing a source close to the investigation, reports, "Investigators believe they have identified the suspected perpetrators of the novichok attack through CCTV and have cross-checked this with records of people who entered the country around that time".
Rowley was taken to hospital on June 30 along with Dawn Sturgess, 44, who died Tuesday.
More than 100 police officers had been searching for the source of Mr Rowley and Ms Sturgess' exposure in Amesbury, where they lived, and Salisbury, where the Skripals were poisoned in March.
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.
Public Health England said Mr Rowley's release does not create a risk to the public.
One of two Britons poisoned by a nerve agent that was used to poison a former Russian agent was discharged from hospital on Friday.
Britain and its allies blamed Russia for the attack in March on the Skripals, prompting the biggest Western expulsion of Russian diplomats since the Cold War.
On Wednesday, global chemical weapons experts completed their investigations in Amesbury, where they sought to identify whether the substance that poisoned the couple was from the same batch used against the Skripals.
The police found no evidence that Rowley and Sturgess ever talked with the ex-spy Sergei Skripal and his daughter Julia.