"The Prosecutor General's Office conducted an additional examination of the quality of the dioxin discovered in the president's blood, and found that limited amounts of this substance were produced... in the U.S., Britain or Russia," Oleksandr Medvedko said.
He said prosecutors were in possession of samples both of Russian and U.S.-made dioxins and will test them.
Medvedko said investigators have established the precise time, venue and circumstances of the poisoning.
"The only thing left is to prove who committed the crime. We are working on that jointly with the Security Service of Ukraine (SBU)," he said.
Yushchenko fell ill shortly after a dinner September 5, 2004 with the then SBU head Ihor Smeshko, and his deputy, Volodymyr Satsiuk, whom he invited to discuss the election campaign.
On September 6, Ukrainian doctors diagnosed food poisoning, and on September 10, as he failed to improve, Yushchenko was taken to the Rudolfinerhaus clinic in Vienna, which he left later in the month for the election campaign.
Medical experts involved, including from the U.S., Austria and Great Britain, were divided over the cause of the illness, but in late 2004 the Rudolfinerhaus clinic finally confirmed that he had indeed been poisoned with dioxin.