Charlie Rowley, who has been in hospital since falling ill in Amesbury on 30 June, is in a critical but stable condition.
When asked if Basu was looking for a "needle in a haystack" and he said, "That's why we need witnesses or intelligence", Sky News reports.
The 45-year old is in critical but stable condition.
Police have said the working theory is that their exposure was linked to the earlier Novichok attack in March on ex-Russian spy Sergei Skripal and his daughter Yulia in the city of Salisbury, who both survived despite extended hospitalisations.
Russian Federation has denied any involvement in the Skripal case and suggested the British security services carried out the attack to stoke anti-Moscow hysteria, an assertion London calls absurd.
"I want to emphasise to everyone in the Salisbury and Amesbury area that nobody, adult or child, should pick up any foreign object which could contain liquid or gel, in the interests of their own safety", she said.
Sturgess died at Salisbury District Hospital, the same facility that nursed the critically ill Skripals.
Police searching for the source of a Russian nerve agent that killed a woman and sickened three others have seized a auto about 40 miles away from the original investigation site.
In a statement, Assistant Commissioner of Scotland Yard Neil Basu said: "I would love to be able to stand here and say how we have identified and caught those responsible and how we are absolutely certain there are no traces of nerve agent left anywhere in the county."The brutal reality, however, is that I can not offer you any such assurances or guarantees at this time".
The investigation became a murder hunt after the death of Sturgess, a 44-year-old mother of three, who lived in Amesbury, a town near Salisbury.
On Tuesday morning, a white Audi was removed from Swindon, about 40 miles from Salisbury, as part of investigations into the novichok poisonings.
Local residents said the paramedic had told them he got bodily fluids on him, but he had been checked over by doctors and given the "all-clear".
REUTERSA auto feared contaminated is taken away.
Residents also claimed he had told them both his vehicle and his clothes were being taken to the government laboratory at Porton Down to be destroyed. "Those involved have the training & expertise to safely remove the vehicle" and the risk to the public remains low".
Basu said he hopes Rowley continues to improve and can give police details about the location of the container.