Transcript for Dec. 4, 2006: Details in Poisoned Spy Case
A mystery surrounding a former Russian spy. Poisoned and killed in London. Has now led investigators beyond the borders of Britain. And back to his homeland in Russia tonight the British government confirmed that investigators have been sent to Moscow and the British home secretary said. They will follow the evidence wherever it goes. That evidence now appears to point increasingly towards Russia's former KGB operation. ABC's senior foreign correspondent Jim -- -- reports from London. -- four British counterterrorism officers arrived in Moscow tonight. Their faces were obscured on British television to conceal their identity. And so they unwittingly became the latest missed three characters. In -- story that delivers a dramatic new twist almost every day. Today it was news that the investigation into Alexander living -- his death was expanding overseas. To Russia. And to the US where this former KGB agent announced he knew who killed in the -- and then promptly disappeared. Dramatic flair or solid new leave. This spy drama never quite sure. The more -- belt about this whole story -- -- the list we know it should because there are so many. Conspiracy theory is basically a -- -- by the -- just by the -- Ten days after his death what we do you know is -- Alexander -- in -- it was killed by a cruel and powerful poison. He was once a real Russian spy. And he had very critical and very worrisome things to say about the Russian government. Today his friends have no doubt his criticism in particular that of Russian president Vladimir Putin. Got him killed. What they do know is that the Russian secret services have been using Alexander's -- as targets in their shooting ranges for. Street before you know trading -- -- and he is was there an enemy number one. To understand how this unassuming -- rear Russian spy ended up an enemy of the State's we have to go back to the beginning. Until the 1990s live and -- it was a loyal agent in the Russian FSB the successor to the KGB. He first raised his profile in 1994. When he was assigned to investigate an assassination attempt. On Russian billionaire Boris Berezovsky. -- for years later he burst into the spotlight when he and other agents including one wearing a mask. Accuse their superiors of plotting to assassinate Berezovsky themselves. -- -- was suspended. Prison. Two years later fled Russia for England where he claimed asylum. Helped by his new outline Berezovsky. Over the next four years in his new home there were other allegations. The -- ago published a book claiming the Russian authorities orchestrated apartment bombings in 1999. That killed 300 people. To help justify -- new invasion of Chechnya. He then spoke publicly about a vast web of Russian spies in London. And in October this year he made another Charles in the east but that's showing up but it journalists forum in London to accused -- Of ordering the murder of Russian journalist and a political -- it is by accusing -- -- that some say he took his criticism to follow. Just two weeks after he accused Tutan in the journalist's murder -- and -- go first fairly you. Doctors identified signs of radiation poisoning but couldn't identify the poison. It was only after he died that they pinpointed the rare and highly radioactive substance polonium 210. But on his final day live in -- took one final shot. Blaming his illness on who else Vladimir Putin. A friend read a letter he'd written. You have shown yourself to have no respect for life -- thirteen. Or and -- the delays and failure. There is no hard proof tying food -- the Russian government to live in the actor's death in the Kremlin has repeatedly denied involvement but. There are signs pointing to Russia. In particular three Russian men who met with -- -- at this hotel. The day that police believe he was poison. Among them was Andre little boy also a former FSB agent. Facing two trips he made to London before the Nikko fell -- They detected radiation both on the planes he traveled on and in the hotels he stating. -- even detected radiation at a soccer stadium where he attended a gain. They are not calling him a suspect. But they will be questioning and Moscow the Deloitte himself has denied involvement and in keeping with the spy drama theme -- claims he's been -- The improvement can -- a quote. I called my lawyers and the next day -- -- breach embassy in Moscow. On my own initiative commitment officials that. I want to be as open as possible with the -- government to stop any speculation. There are other theories British police are examining the possibility rogue agents of the Russian security services killed him. And some believe -- had nothing to do with -- but suspect Russian Nationalists instead. Attempting to drive the country the confrontation with the west. So they want to test which and they want to pretend to disassociate himself. From the of -- -- of the nineties. And the west. And do -- old style. Old school. Yet anywhere -- the main weapon that is to -- America. And there are other possibilities but most observers do agree on is that it took vast expertise and resource and carry off such an elaborate crying. Should mean that there are either coordinated from the Kremlin. Or that we have a some kind of parallel government and Russia can control and perform such a sophisticated operation. Involving nuclear materials. May have won -- that doesn't fit the plot line. And that's that this crime. May never be solved. I'm -- shoot -- for Nightline in London.
This transcript has been automatically generated and may not be 100% accurate.