Kiev, Ukraine ? Ukrainian prosecutors Sunday reopened their investigation into allegations Viktor Yushchenko was poisoned after doctors treating the opposition leader confirmed he had been slipped the toxic chemical dioxin.

Yushchenko returned home to campaign for this month’s presidential runoff vote. He said he did not want the poisoning issue to overshadow the Dec. 26 election, but the director of Vienna’s elite Rudolfiner clinic said a potential criminal case could be involved.

“We are not dealing with simple pimples. We are dealing with a poisoning and the suspicion of third-party involvement,” Dr. Michael Zimpfer said, referring to the disfigurement of Yushchenko’s face.

Clinic doctors said it took a newly developed test, conducted by a lab in Amsterdam, Netherlands, to determine beyond a doubt that dioxin poisoning caused Yushchenko’s mystery illness in September, leaving him disfigured and in pain.

Whoever was responsible may have thought dioxin was untraceable, Zimpfer said.

“Until recently, there has been no (blood) testing available” for dioxin, Zimpfer said. “This may be one of the reasons that this kind of poisoning, if it was a criminal act, was chosen.”

Yushchenko said he expected an investigation to find the culprits, but he urged that it be conducted after the runoff to avoid influencing the results.

“I don’t want this factor to influence the election in some way — either as a plus or a minus,” Yushchenko said in Russian as he left the clinic and headed back to Kiev. “This question will require a great deal of time and serious investigation. Let us do it after the election — today is not the moment.”