Of course they've already tried before...
Yushchenko: Russia blocking poisoning probe
By Bonnie Malkin and agencies, September 12, 2007
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) [was] the third and [former] 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