Authorities conducted extensive forensic tests looking for the source of a nerve agent that made ill two people thought to have handled a contaminated item from the March attack on a Russian ex-spy and his daughter.
A police officer also underwent a precautionary test at a hospital to check for possible contamination related to the case, but Wiltshire Police said that he had been cleared.
The man and woman poisoned a week ago are in critical condition at Salisbury District Hospital, which is also where Sergei and Yulia Skripal spent months being treated after they were poisoned.
Authorities have said all four were made ill by Novichok, a nerve agent weapon developed by the Soviet Union during the Cold War.
Police think 44-year-old Dawn Sturgess and her partner, 45-year-old Charley Rowley, had secondary exposure to the chemical weapon used in the attack on the Skripals.