Moscow appeared defiant Tuesday as it faced a looming British deadline to explain the nerve-agent poisoning of a former spy, demanding London provide a sample of the substance used in the attack and open up its probe to Russian officials.

U.K. Prime Minister Theresa May has said it is highly likely Russia was behind the poisoning of former double agentSergei Skripal and his daughter, and threatened retaliation in the case, marking a new low point in already strained post-Cold War relations.

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