In 1978, Georgi Markov, a Bulgarian dissident, was said to have been jabbed with a poison-tipped umbrella in London with the connivance of the K.G.B. The Federal Security Service, the K.G.B.’s successor, was implicated in subsequent poisonings of Mr. Putin’s critics.

Mr. Litvinenko was investigating the death of Anna Politkovskaya, a Russian journalist, who was shot to death in her Moscow apartment building the month before he died.

In 2016, after a protracted demand for answers from Mr. Litvinenko’s widow, Judge Robert Owen, who had since retired from the British High Court, delivered an unequivocal finding: “I am sure that Mr. Lugovoy and Mr. Kovtun placed the polonium 210 in the teapot at the Pine Bar on 1 November 2006. I am sure that they did this with the intention of poisoning Mr. Litvinenko.”

Judge Owen also concluded that the evidence “establishes a strong circumstantial case that the Russian State was responsible for Mr. Litvinenko’s death,” and that the security service’s operation was “probably approved” by Mr. Putin.

Last year, the European Court of Human Rights ruled that Russia was responsible for the killing.

Dmitry Vladimirovich Kovtun was born into a military family on Sept. 25, 1965, in Moscow. In the 1980s, he attended the Moscow Higher Military Command School, where his classmates included Mr. Lugovoi. After they graduated, they served in the K.G.B.’s ninth directorate, which was responsible for protecting the Kremlin’s top brass.

While Mr. Lugovoi became a public figure as a member of the Russian parliament, much less is known about Mr. Kovtun’s career path after the Soviet Union collapsed.

He and his German-born wife, Inna Hohne, moved to Hamburg, where he later claimed political asylum. The couple separated, and he returned to Moscow as a business consultant after he had worked as a waiter and, Ms. Hohne was quoted as saying, aspired to act in pornographic movies.

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