Ukrainian prosecutors Sunday reopened their investigation into allegations Viktor Yushchenko (search) was poisoned after doctors treating the opposition leader confirmed he had been slipped the toxic chemical dioxin (search).

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 (search) 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 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."

Later, after returning to Kiev, Yushchenko said "soon we'll know who did it."

Tests showed the toxin was taken orally after likely being slipped into something Yushchenko ate or drank.

"This is the first case internationally where the intake has been oral. Usually it's inhaled, it's very different," Zimpfer said.

The Amsterdam tests found Yushchenko's blood contained more than 1,000 times the normal amount of dioxin.

Ukraine's prosecutor general's office said it reopened the criminal investigation it closed in November for lack of evidence. Yushchenko fell ill Sept. 5 and was treated at the Vienna clinic twice before.

Lawmakers from Yushchenko's party said the clinic findings confirmed that opponents wanted to assassinate or disable the Western-leaning politician rather than take the risk he would defeat Prime Minister Viktor Yanukovych in the presidential election.

Yanukovych campaigners rejected suggestions the Kremlin-backed prime minister was involved in a poisoning attempt. There is "no logic in such an accusation," said Taras Chornovyl, Yanukovych's campaign manager.

Ukraine's Supreme Court ordered the runoff after ruling that fraud in the Nov. 21 runoff gave the election to Yanukovych, the hand-picked candidate of outgoing President Leonid Kuchma.

Parliament also has adopted electoral changes aimed at preventing fraud in return for handing over some presidential powers to lawmakers, easing political tensions in the former Soviet republic.

Dioxin is a byproduct of industrial processes such as waste incineration and chemical and pesticide manufacturing, and the chemical is known to cause cancer.

The massive quantities found in Yushchenko's system caused chloracne, a type of adult acne caused by exposure to toxic chemicals. The condition is treatable but can take up to three years to heal.

Zimpfer said Yushchenko's treatment will now be "very difficult and long."

While high concentrations of dioxin remain in Yushchenko's blood, doctors said his organs have not been damaged and he is fit for the campaign trail.

"He has almost made a complete recovery," Zimpfer told The Associated Press. "His liver is fine, his pancreas is fine, but he still has residual pain" and is taking painkillers.

Dr. Nikolai Korpan, the physician treating Yushchenko, said it was too early to tell what other health problems he might develop.

For now, he said, "we can confirm that his health is very good at this moment and he can do his job," Korpan said.

The 50-year-old opposition leader thanked the medical staff as he checked out of the clinic.

"They've spent many days and nights with me and I am very happy to be alive in this world today," Yushchenko said, with his American-born wife, Kateryna Chumachenko, translating. "I thank these people for this."

Yushchenko praised the thousands in Ukraine who staged street protests against the outcome of the Nov. 21 runoff.

"We haven't seen anything like that for the past 100 years," he said. "I think it would be appropriate to compare this to the fall of the Soviet Union or the fall of the Berlin Wall."

Yushchenko wants to move his former Soviet republic closer to the West politically and is backed largely by the Ukrainian-speakers who want to end what they call been mass corruption during the previous decade.

Yanukovych draws his strength from the Russian-speaking, industrial east, which accounts for one-sixth of Ukraine's population of 48 million.

Also Sunday, Yanukovych's spokesman, Oleh Ternovsky, said the prime minister wanted Ukraine's parliament to investigate whether the United States helped finance Yushchenko's campaign.

Washington has spent more than $65 million in the past two years to aid political organizations in Ukraine, but U.S. officials say no American funds were sent directly to political parties.