Metropolitan Police Assistant Commissioner Neil Basu said the death of Sturgess on Sunday (Monday NZT) shows that she and Rowley were exposed to a large quantity of Novichok, a military-grade nerve agent produced in the Soviet Union during the Cold War.
The London police anti-terrorism department earlier stated that Sturgess and Rowley came into contact with Novichok - the same nerve agent that poisoned former Colonel of the Main Intelligence Directorate Sergei Skripal and his daughter Yulia this past March.
Britain blamed the Russian state for the attack on Sergei Skripal and his 33-year-old daughter - an allegation Moscow has repeatedly denied.
Chief Medical Officer Dame Sally Davies said people in the Amesbury and Salisbury area should not "pick up any foreign object which could contain liquid or gel, in the interests of their own safety".
He is in "a critical but stable condition and is now conscious", a spokesperson for Salisbury Hospital has said.
The UK government has blamed Russian Federation for the incident, but the authorities in the Motherland deny any involvement.
"I would need a forensic link to be definitive, but 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".
The wide investigation is now a homicide inquiry.
"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", Basu added.
"The brutal reality, however, is that I can not offer you any such assurances or guarantees at this time".
Police now believe the two handled a contaminated item with a high dose of Novichok, although they have not been able to confirm if it was from the same batch as the one used in the attempted murder of the Skripals. "We will not be providing further commentary around our contact with Charlie".
Police officers guard a cordon in Salisbury, one of five locations in the city and Amesbury under investigation.
The UK's top anti-terror police office said on Wednesday that he could not guarantee those responsible for the poisonings will be caught.