The pair were poisoned March 4 after being exposed to what British police believe was a military-grade nerve agent.
Counterterrorism detectives will continue to focus their inquiries on the home address for the coming weeks, and possibly months, after the father and daughter were found unconscious on a park bench in Salisbury earlier this month.
"At this point in our investigation, we believe the Skripals first came into contact with the nerve agent from their front door", said Dean Haydon from London's Metropolitan Police. They had no visible injuries, according to police.
Police said around 250 counter-terrorism detectives are now working on the case, which could continue for months.
Officers are examining more than 5,000 hours of closed-circuit television and more than 1,350 seized items, police said.
Michigan State official arrested
Mr Strampel was accused also of grabbing two students' buttocks at the college's annual ball and a scholarship dinner. As well as being a dean, he was the lead medical director of MSU HealthTeam, according to the school's website .
The OPCW also obtained a court ruling on Friday allowing them to take blood samples from the two victims.
Scotland Yard added that the "highest concentration of the nerve agent" had been discovered on the front door of Col Skripal's home in a cul-de-sac close to Salisbury city centre.
Germany will expel four Russian diplomats over Moscow's suspected involvement in the poisoning of former Russian double agent Sergei Skripal in England, Germany's Sueddeutsche Zeitung reported on Monday. Russian Federation denies it was involved and has suggested the United Kingdom could be behind it.
Moscow has already ordered 23 British diplomats to leave Russian Federation in response to the same move by London, and is expected to announce further retaliatory measures against other countries. Those countries include the United States, Canada, Australia and 18 European Union states. Britain expelled 23 Russians as a result and Moscow retaliated by expelling the same number of Britons.