Yushchenko said the poisoning nearly killed him and suggested Russia may have been behind the "attempted murder." The Kremlin has denied any involvement.
A source at the Prosecutor's Office in Kiev said Yushchenko had continually refused to give blood tests and offered little cooperation in the official investigation.
There may be two reasons for this, the unnamed source told the Ukrainian daily Sehodnya, one being that Yushchenko used the poisoning scandal as a ploy to win popular support.
"Either Yushchenko does not want to answer the questions that would show that his original testimony was false, or he does not want to implicate the people responsible for thinking up this dioxin 'special operation' for the sake of a political moment," the source said.
"Either way, he could be charged."
In May, Ukrainian Prosecutor General Viktor Pshonka threatened to close the investigation if Yushchenko did not cooperate.
Yushchenko, whose health has improved significantly since 2004, said in January he wanted to "close that chapter" in his life.
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