LONDON — In the dank, dark days of November 2006, as Alexander V. Litvinenko, a former K.G.B. officer turned foe of the Kremlin, lay dying in a London hospital, he and his associates composed a deathbed missive to President Vladimir V. Putin.

In the letter, Mr. Litvinenko said he could hear “the beating of wings of the angel of death” and blamed Mr. Putin for his plight. But, he told the Russian leader, “the howl of protest from around the world will reverberate, Mr. Putin, in your ears for the rest of your life.”

Image Mr. Litvinenko in 2002. Credit... Alistair Fuller/Associated Press

The echo could be heard Thursday with the release of the final report of a lengthy public inquiry into Mr. Litvinenko’s death. It was probable, said the report, by a retired judge, Sir Robert Owen, that Mr. Putin and his spy chief at the time, Nikolai Patrushev, had approved an operation to kill Mr. Litvinenko, using a highly toxic and rare isotope, polonium 210.