LONDON -- The nerve agent Novichok could remain active for 50 years if kept in a sealed container, Britain's top counterterrorism police officer said Wednesday, adding that he cannot yet "guarantee" there are no traces of the lethal poison in southwestern England. Metropolitan Police Assistant Police Commissioner Neil Basu told residents of Amesbury Tuesday night that police are searching for the container that held the nerve agent believed to have poisoned two people on June 30.

"I would love to be able to say that we have identified and caught the people responsible and how we are certain there are no traces of nerve agent left anywhere in Wiltshire," he said.

"But the brutal reality is that I cannot offer you any reassurance or guarantee at this time."

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He said there is so far no definitive forensic proof that the Novichok that poisoned 44-year-old Dawn Sturgess and 45-year-old Charlie Rowley was the same batch used in March against ex-Russian spy Sergei Skripal and his daughter Yulia. Basu said this can only be proved by scientists conducting detailed analysis, but that any other explanation is extremely unlikely.

"This is a very rare substance banned by the international community and for there to be two separate, distinct incidents in one small English county is implausible to say the least," he said.

The nerve agent was produced in the Soviet Union during the Cold War. Britain has accused the Russian state of the attack on the Skripals, a charge denied by the Kremlin.

Sturgess died on Sunday; officials say Rowley has shown slight but significant improvement and has recovered consciousness. He is in critical but stable condition and officials warn he is not yet out of danger.

Basu said he hopes Rowley continues to improve and can give police details about the location of the container. He said it is possible Sturgess and Rowley had the container in their possession for some time before opening it with disastrous results.

"The brutal fact is we don't know where they found it. I am hoping Charlie recovers and when he recovers he will be able to tell us and perhaps shed some light on it which will narrow our search dramatically. There is a possibility they found it on March 5 and only opened it in the past 10 days," he said.