Secretary of State Rex Tillerson said Monday a poison used against a former Russian spy in England "clearly came from Russia."
He said the nerve agent couldn't have come from any other country but that he's not sure if Russian President Vladimir Putin knew about the attack.
Reports out last week said the nerve agent left both former double agent Sergei Skripal and his daughter in critical condition.
U.K. Prime Minister Theresa May said Monday it was "highly likely" Russia was behind the attack.
Russia has denied any role in the poisonings.