The British police say that they believe former Russian double agent Sergei Skripal and his daughter first came into contact with a nerve agent at their home.
The police have identified the highest concentration of the nerve agent used against Sergei and Yulia Skripal on their front door, reported CNN.
Retired military intelligence officer Skripal, 66, and his daughter, Yulia, 33, were found slumped on a bench in Salisbury city centre on March 4.
The poison attack left Skripal and his daughter Yulia comatose on a park bench.
Several media reports stated that About 250 counter-terrorism officers continue to work on the case, the authorities have reviewed over 5000 hours of surveillance.
They have identified around 500 witnesses and taken hundreds of statements.
Britain holds Russia responsible for the attack, a move which is likely to plunge the bilateral relationship between the two countries to a new role.
More than 20 other countries have kicked out Russian diplomats. Those countries include the United States, Canada, Australia and 18 European Union states.