Former Russian spy Sergei Skripal and his daughter Yulia likely came into contact with the toxic nerve agent Novichok at the front door of their house, according to British police.
A police spokesperson said traces of the deadly substance had been found in other areas nearby but officers were concentrating their investigation around the home.
The risk to the public remained low, they added.
Skripal, 66, and Yulia, 33, were found collapsed on a bench in Salisbury, England, after visiting a restaurant in the town centre on March 4. Both remain in critical condition.
The restaurant was eventually closed down and the ward they were treated in evacuated as evidence of their poisoning became clear.
The investigation had become "one of the largest and most complex investigations undertaken by British counterterrorism policing" and is likely to take many months, according to authorities.
Moscow has strongly denied any involvement in the incident, which has led to the expulsion of Russian diplomats by the UK and its allies.
British foreign secretary Boris Johnson said Russia underestimated the international response to the attack.