A “very small amount” of liquid nerve agent was used in the March 4 poisoning of Sergei V. Skripal and his daughter, and it did not release gases or vapors, British environmental scientists said on Tuesday.

The announcement came as the police prepared to withdraw from nine operational crime scenes in the medieval city of Salisbury and hand them over to scientists for a meticulous decontamination program.

Already, tourists visiting Stonehenge and Salisbury’s 13th-century cathedral have gawked at crime-scene tape surrounding a series of sites that the two Russians visited on the day they were found collapsed on a bench beside the Avon River.

A decontamination process could go on for “a number of months” and promises to be a vast undertaking, requiring backup from 190 army and air force specialists as well as input from the Defense Science and Technology Laboratory, the Home Office and the Ministry of Defense, the authorities said.