(CBS) When former Soviet KGB agent Alexander Litvinenko died last November, it quickly got the world's attention.

The way he died, and the twists and turns that unfolded before his death, were something right out of a spy novel.

None of the friends and family who knew Litvinenko as "Sasha" could have predicted the discovery made as he was quarantined in a London hospital in his final hours — that his organs were failing because of exposure to lethal radiation from polonium-210.

Ever since Litvinenko broke with Russia's secret service, known as the FSB, and fled Russia in 2000, he said he was a marked man. He had exposed too many secrets.

Now, a trail of radiation left by one of the world's deadliest substances was revealing its own clues, which led to a London sushi bar, and a cup of tea, and to the London offices of Boris Berezovsky, a fellow émigré and Litvinenko patron who built a fortune during the breakup of the Soviet Union.

The radiation trail also led to Russia, where most of the world's polonium is manufactured, and to another ex-spy, Andrei Lugovoi, charged in absentia by Britain with Litvinenko's murder.

Lugovoi accuses British intelligence and Berezovsky with murdering Litvinenko to discredit Russia's president, Vladimir Putin.

But Litvinenko's final statement, read just after he died, blames Putin for his violent death and for untold suffering in Russia.

Litvinenko's widow, Marina Litvinenko, and his close friend, Alex Goldfarb, have written a book called "Death of a Dissident: The Poisoning of Alexander Litvinenko and the Return of the KGB."

They visited The Early Show Tuesday to discuss the book, and the case.

The book has the full story of Litvinenko's life and death, from what he was investigating to their views on who killed him and why someone wanted him dead. It names names, explains why Litvinenko broke with Moscow and revealed some of the biggest secrets of Russia's secret service.

"After what happened to Sasha," Goldfarb said to co-anchor Harry Smith, "somebody has to tell the story."

He said the trail left by the polonium used to poison Litvinenko shows that Lugovoi was the murderer.

Goldfarb added that nuclear experts tell him the polonium came from a Russian reactor. He says it was chosen because it's almost impossible to trace and it was just by chance that British authorities realized it was in play. "Had they not uncovered it, it would be an unexplained death," he said.

"We have reasons to believe it was the (Russian government behind Litvinenko's death), and Sasha himself said it was Putin," Goldfarb said, "because he knows the man personally ... and this describes the kind of regime it is."

To see the Early Show segment, click here.

To read an excerpt of "Death of a Dissident," click here.

"Death of a Dissident" is published by The Free Press, which is part of Simon & Schuster, which is owned by CBS, as is CBSNews.com.

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