LONDON -- British police said Saturday that search teams have found over 400 items as part of the investigation into the poisoning of two Britons by the nerve agent Novichok -- but searches will likely take many more weeks as officers seek further evidence.

Metropolitan Police announced a breakthrough in the case Friday when they said they had found a small bottle believed to be the source of the nerve agent that killed Dawn Sturgess and sickened Charlie Rowley. The bottle was found at Rowley's home in Amesbury, a southwestern town not far from Salisbury, where British authorities say Russian ex-spy Sergei Skripal and his daughter Yulia were poisoned with Novichok in March.

Britain blames the Russian government for the March attack, an accusation the Kremlin has denied. The case prompted Western nations including the United States and Britain to expel scores of Russian diplomats and for Russia to retaliate with similar expulsions.

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Police are trying to figure out whether the substance in the bottle — confirmed by scientists as Novichok — came from the same batch used in the attack against the Skripals. They're also looking into where the bottle came from and how it got into Rowley's house.

"It is not an exaggeration to say that the search process linked with both this and the Salisbury investigation has been one of the most complex and difficult that U.K. policing has ever faced," said Assistant Police Commissioner Neil Basu, Britain's top counterterrorism officer.

The force said, in total, search teams recovered over 400 "exhibits, samples and items" linked to the investigation into the poisoning of Sturgess and Rowley. It said a "significant number" of the items are potentially contaminated and have been sent to laboratories for analysis.

Sturgess, 44, and Rowley, 45, were sickened on June 30. Sturgess died in a hospital on July 8. Rowley was in critical condition for more than a week, but has regained consciousness.

Police said earlier they suspected the pair had handled a container contaminated with Novichok and had no reason to think they were targeted deliberately.

In a statement detailing the difficulties police face over the probe, Basu said each search has to be meticulously planned to ensure that traces of the deadly agent don't get leaked out. Protective suits for each officer take 40 minutes to put on and take off, and they can only work in short bursts because of heat and exhaustion.

"Not only are we trying to solve an extremely serious crime that has been committed, but we're also working to identify any potential outstanding risks to the public; all whilst ensuring that all those involved in the search process are not themselves exposed to any risk of contamination," he said.

Officials say Novichok, produced by the Soviet Union during the Cold War, could remain active for 50 years if kept in a sealed container

The Skripals survived and were released from a Salisbury hospital before Rowley and Sturgess were poisoned and taken there. British authorities have taken the Skripals to a secret protected location for their safety.