The surviving victim of the Amesbury novichok poisoning has regained consciousness and experienced a "small but significant improvement to his condition", a spokeswoman for Salisbury district hospital said.
In a statement released through the Met police, Sturgess's family said her death in hospital on Sunday, after falling ill along with her partner Charlie Rowley on June 30, "has been devastating for us".
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. "Charlie is still very unwell and will continue to require specialist, round-the-clock care here at Salisbury District Hospital".
It killed Sturgess, 44, who died on Sunday evening.
A woman who died after being poisoned with a nerve agent that also struck a former Russian spy in March must have handled a contaminated item, and tracking it down is key to police investigations, Britain's top counter-terrorism officer said.
"I would need a forensic link to be definitive, but this is a very rare substance banned by the worldwide community and for there to be two separate distinct incidents in one, small English county is implausible to say the least".
"This is a very rare substance banned by the global community and for there to be two separate, distinct incidents in one small English county is implausible to say the least", Basu added.
A police officer stands at a cordon around a public litter bin next to a supported housing project in Salisbury on July 5 after two people were exposed to Novichok nerve agent.
"The brutal reality, however, is that I can not offer you any such assurances or guarantees at this time".
He said it is possible Sturgess and Rowley had the container in their possession for some time before opening it with disastrous results.
Detailed searches are continuing at Sturgess's address at John Baker House, Salisbury and Rowley's home in Muggleton Road, Amesbury, along with Queen Elizabeth Gardens in Salisbury.
"The brutal fact is we don't know where they found it".
British Prime Minister Theresa May said she was appalled and shocked by Sturgess's death and the interior minister, Sajid Javid, said the "desperately sad news only strengthens our resolve to find out exactly what has happened".
Public Health England have said the risk to the public following the incident is low.
All nerve agents poison in the same way, which is to inhibit the enzyme involved in regulating the body's messages along nerves and between nerves and muscle.
How badly affected people are varies greatly - for example, some people are able to break down the chemicals three times as fast as others.
'The temperature inside the forensic tents two days ago was 40 degrees - before they put the suits on.