Officers believe several Russians were involved in the attempted murder and say they are looking for more than one suspect.
Investigators believe they have identified the suspected perpetrators of the Novichok attack through CCTV and have cross-checked this with records of people who entered the country around that time, a source close to the investigation said.
They said investigators were sure the suspects were Russian.
The news comes after Dawn Sturgess and her partner Charlie Rowley fell victim to novichok believed to be from the same batch as that used to target the Skripals.
Ms Sturgess, 44, died eight days after being exposed to 10 times the amount of nerve agent the former double agent and his daughter came into contact with.
Investigators believe the substance was in a discarded perfume bottle that the couple found in a park, the source said, adding that Ms Sturgess likely sprayed the substance directly onto her skin.
News Source SkyNews