The nerve agent used to attack former Russian double agent Sergei Skripal and his daughter was delivered in a "liquid form", the Department for Environment, Food and Rural Affairs (Defra) has said.
Only a "very small amount" of the deadly Novichok substance was used against Mr Skripal, 66, and 33-year-old Yulia, it is understood.
The highest concentration was found at Mr Skripal's home on the outskirts of Salisbury, with eight other areas across the city potentially contaminated.
Further details about the substance emerged at a press briefing on Tuesday.
A spokesman for Defra said: "In this instance, direct contact is required for a person to poisoned. Only a small proportion of the material is transferred in each contact and the substance is diluted in each secondary or tertiary contact.
"The class of nerve agent does not produce significant vapour or gas and can only be moved between sites by direct transfer from a contaminated person or by moving a contaminated item."
Asked what form the nerve agent was in, the Defra spokesman said: "It's in a liquid form."
It comes as the first of 10 sites cordoned off across the city was reopened to the public on Tuesday.
Tests confirmed the area of London Road cemetery, which contains the remains of Mr Skripal's dead wife and son, was not contaminated.
Meanwhile, work is set to begin to decontaminate the nine other locations experts either know or believe are contaminated.
"We either know there is contamination there, or we think there is a probability we could find contamination there," Defra said.
A multimillion-pound operation, involving around 190 specialist military personnel, is expected to start in the coming days, with the process lasting months.
It will first focus on two areas near Bourne Hill police station and two ambulance stations, before work starts on The Maltings area - where the Skripals were found - Zizzi restaurant, a car compound and the home of poisoned police officer Nick Bailey.
The Mill pub and Mr Skripal's home, which are still part of the police investigation, will be the last to be cleaned.