It is beyond the realm of possibility that the U.S. would ever commit such an act against their own innocent population. However, when it comes to Russia and their KGB/FSB, just about anything is possible. They have a long history of terrorism and murder. Scores of reporters critical of Putin have been mysteriously murdered in Russia. They attempted to murder the president of the Ukraine by poisoning him. They murdered by poisoning Alexander Litvinenko, a former KGB agent who was 'spilling the beans' on what they were up to (staging and/or contributing to terror attacks in order to later justify a brutal 'retaliation' vs the opposition groups).
You are naive in the extreme if you believe the Russians aren't capable of this. I wouldn't be surprised if they initiated the Bush-911 conspiracy theories in order to draw attention away from themselves and make similar theories regarding them seem just as 'off the wall' and inconceivable, at least in the minds of those who know zilch about the history of the KGB (most people, unfortunately).
Mr Yushchenko before and after the poisoning
"Mr Yushchenko, a pro-European politician who wanted to bring his country out of Russia's shadow, fell seriously ill on September 6, 2004 as he was competing in presidential elections against a pro-Moscow candidate, Viktor Yanukovich, now prime minister.
After months of tests in an Austrian clinic, it was determined that he had ingested a massive amount of the poison dioxin.
Although he survived, his face was left bloated and pockmarked, and he has had to undergo regular treatment to rid his body of the toxin.
In an interview with Le Figaro he said he believed the dioxin used to disfigure him was made in a Russian lab.
Mr Yushchenko did not directly accuse the Russian government of being behind his poisoning, but he did say he had 'practically put all the pieces together' and the attempt against him 'was not a private action'. ..."
http://www.telegraph.co.uk/news/worldnews/1562838/Yushchenko-Russia-blocking-poisoning-probe.html
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"Viktor Andriyovych Yushchenko (born February 23, 1954) is the third and current President of Ukraine". He took office on January 23, 2005.
http://en.wikipedia.org/wiki/Viktor_Yushchenko
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(Ukraine) Hunt starts for Yushchenko's poisoner:
http://www.telegraph.co.uk/news/worldnews/europe/ukraine/1478922/Hunt-starts-for-Yushchenko%27s-poisoner.html