British police said they believe former double agent Sergei Skripal and his daughter likely first came into contact with a military-grade nerve agent at their front door.
Deputy Assistant Commissioner Dean Haydon said in a statement late on March 28 thatpolice are now focusing their investigation in and around Skripal's home.
London's Metropolitan Police said on March 28 that they have "identified the highest concentration of the nerve agent, to-date, as being on the front door of the address."
It's the first time British police have suggested where the Skripals could have been poisoned. Police have also searched a variety of sites around the southern English city of Salisbury, including a pub, a restaurant and a cemetery.
Ties between London and Moscow are badly strained by the poisoning of Skripal and his daughter, Yulia, who remain in critical condition after being exposed to a powerful nerve toxin in Salisbury on March 4.
Britain blames Russia for the poisoning, triggering a diplomatic dispute between Moscow and Western countries.
Russia has called on Britain to prove it did not poison Skripal, saying that unless it sees proof it will consider the incident an attempt on the lives of Russian citizens.
"An analysis of all the circumstances...leads us to think of the possible involvement of the British intelligence services," the Russian Foreign Ministry said in a statement on March 28.
"If convincing evidence to the contrary is not presented to the Russian side, we will consider that we are dealing with an attempt on the lives of our fellow citizens as the result of a massive political provocation."
The statement came as more than two dozen governments around the world and the NATO military alliance are expelling Russian diplomats in response to the poisoning of Sergei and Yulia Skripal.
The expulsion of more than 150 Russians by 28 countries -- including Britain, the United States, and most members of the European Union -- dwarfs similar measures taken during Cold War spying disputes.
Montenegro on March 28 became the latest country to announce the expulsion of a Russian diplomat over the poisoning.
The government of Montenegro, which is NATO's newest member and also aspires to join the EU, said the decision was made in line with steps undertaken by its "allies and partners, the EU and NATO," and in solidarity with Britain.
Montenegro also revoked its consent for Russia's honorary consul.
Britain a day earlier hailed the mass expulsions as a "turning point," while Russia asserted that it was the result of "colossal" pressure by the United States and vowed to respond to the move.
Valentina Matviyenko, the speaker of Russia's Federation Council -- the upper house of parliament -- said on March 28 that Moscow will respond to the expulsions in kind.
"Without a doubt, Russia, as is diplomatic practice, will respond symmetrically and observe parity when it comes to the number of diplomats," she told reporters.
Asked about a response to the U.S. expulsions in particular, Deputy Foreign Minister Sergei Ryabkov said that "all options are on the table."
"We should analyze the entire situation in totality, assess the degree of hostility demonstrated by the policies of Washington and London," and choose an appropriate response, Ryabkov said.
Britain ordered the expulsion of 23 Russian diplomats suspected to be spies on March 14, after Moscow ignored Prime Minister Theresa May's demand for an explanation of how a military-grade nerve agent developed in the Soviet Union came to be used in England. May said at the time that there is "no alternative conclusion, other than that the Russian state was culpable for the attempted murder" of Sergei and Yulia Skripal "and for threatening the lives of other British citizens in Salisbury.
Other countries followed suit, with Washington ordering out 60 Russian diplomats suspected of being spies less than a week after President Donald Trump congratulated Putin on his reelection. Germany, France, and Poland each expelled four diplomats; the Czech Republic and Lithuania each expelled three; Spain, the Netherlands, Denmark, and Italy each expelled two; and Belgium, Hungary, Ireland, Sweden, Croatia, Romania, Finland, Latvia, and Estonia announced they were expelling one Russian diplomat each.
Outside the EU, Canada, Australia, Ukraine, Macedonia, Moldova, and Norway also announced expulsions.
Sergei Skripal, a former Russian military intelligence officer imprisoned by Moscow after being convicted of passing on information about Russian agents in various European countries, came to Britain in a 2010 spy swap. He and his daughter were found slumped on a bench in Salisbury on March 4, a day after she arrived from Moscow to visit her father.