LONDON (AP) - British detectives investigating the poisoning of two people by the nerve agent Novichok in southwestern England said Friday that a small bottle found in the home of one of the victims tested positive for the deadly substance.

Dawn Sturgess, 44, and Charlie Rowley, 45, were sickened on June 30 in a town not far from Salisbury, where British authorities say a Russian ex-spy and his daughter were poisoned with Novichok in March. Sturgess died in a hospital on Sunday. Her partner, Charlie Rowley, initially was in critical condition, but has regained consciousness.

The Metropolitan Police said the small bottle was found during searches of Rowley’s house Wednesday and scientists confirmed the substance in the bottle was Novichok. Police have interviewed Rowley in recent days since he recovered consciousness.

Police are still looking into where the bottle came from and how it came to be in Rowley’s house. Further tests will be carried out to establish whether it is from the same batch that was used to poison Russian ex-spy Sergei Skripal and his daughter in March.

Assistant Police Commissioner Neil Basu said no more details would be provided about the bottle. More than 100 police officers had been searching for the source of Rowley and Sturgess’ exposure in the towns of Amesbury, where they lived, and Salisbury, where the Skripals were poisoned.

Basu said cordons would remain in place in some locations to protect the public despite the apparent breakthrough in the case.

“This is clearly a significant and positive development. However, we cannot guarantee that there isn’t any more of the substance left,” Basu said. “This is to allow thorough searches to continue as a precautionary measure for public safety and to assist the investigation team.”

Britain’s Foreign Office said Friday that the U.K. has asked the Organization for the Prohibition of Chemical Weapons to collect sample for analysis at its labs. The organization has the power to assign blame for chemical weapons use.

The Novichok saga began in March when Sergei and Yulia Skripal mysteriously fell ill on a park bench in Salisbury. They were found to have been poisoned with Novichok, a lethal nerve agent produced in the Soviet Union during the Cold War.

Prime Minister Theresa May blamed the Russian government for the attack, which the Kremlin has vehemently denied.

The Skripals survived and have been released from a hospital, but are in a secret protected location.

The case led the United States and other countries to expel a large number of Russian diplomats.

Public health officials say the risk to the public is low, but advised people not to pick up any strange items.

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