"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." One victim, Dawn Sturgess, has died.
Basu says he hopes her partner Charlie Rowley improves and can give police details about the location of the container.
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