A police cordon put around a park in Salisbury where traces of the nerve agent Novichok were found has been lifted.
Queen Elizabeth Gardens in Salisbury was sealed off in July.
In March, Sergei Skripal, 66, and his daughter Yulia, 33, were found collapsed on a bench in the Maltings shopping centre, near the gardens.
In June, Dawn Sturgess and Charlie Rowley fell ill after they visited the gardens. Ms Sturgess later died.
Police said the site was now safe and could be returned to public use.
Nearby streams and rivers were also searched as part of the investigation.
Deputy Chief Constable Paul Mills said: "Today marks a milestone in terms of our ongoing response to the incident.
"Queen Elizabeth Gardens has been searched by specially-trained officers and the results reviewed.
"Decontamination activity was conducted, is now complete and the site is safe and can be returned to public use."
The Skripals were found unconscious suffering from the effects of the nerve agent on 4 March.
Ms Skripal and her father were discharged from Salisbury District Hospital in April and May respectively, following treatment.
Ms Sturgess died after she and Mr Rowley collapsed having been exposed to the nerve agent on 30 June.
Meanwhile, the government has pledged another £2.5m in funding for Wiltshire Police to cover the costs of the two cases.