A former Russian spy and his daughter may have first come into contact with a military-grade nerve agent at the front door of their home, U.K. police said Wednesday.

Counterterrorism police announced that their investigation into the March 4 poisoning of Sergei Skripal and his daughter Yulia is focused on their home in Salisbury, England.

It was around the front door that investigators discovered the highest concentration of the nerve agent yet, police said. There remains a low risk to neighbors of the Skripals.

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The Skripals remain hospitalized in critical condition. In an interview with the BBC this week, Viktoria Skripal, the niece of Sergei Skripal, said the ex-spy and his daughter have a slim chance of survival, adding that the prognosis "really isn’t good."

"Out of 99 percent I have maybe 1 percent of hope," she said. "Whatever it was has given them a very small chance of survival. But they’re going to be invalids for the rest of their lives."

British officials have blamed the poisoning on the Russian government, pointing to the fact that the attack was carried out with a Soviet-era Novichok nerve agent as evidence. Moscow has denied any role in the poisoning.

Still, the episode has sent tensions between Russia and the West soaring. More than two dozen countries moved this week to expel Russian officials from diplomatic outposts on their soil in retaliation for the poisonings.

President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE joined those countries on Monday, booting 60 Russian officials out of the U.S. and shuttering the Russian consulate in Seattle.

Moscow has vowed retaliation for the diplomatic expulsions, which it has blamed on a "colossal blackmail" campaign by the U.S.