Poland's Prime Minister said that "huge amounts of chemical waste" were probably dumped intentionally into the Oder River, which runs along the border with Germany, causing environmental damage so severe it will take the river years to recover.
Tons of dead fish have been seen floating or washed ashore on the Oder's banks over the past two weeks but the issue only erupted into a major scandal late this week.
Prime Minister Mateusz Morawiecki, whose government is under pressure for its handling of what appears to be a major environmental catastrophe, vowed that Polish authorities would hold the perpetrators to account.
"Huge amounts of chemical waste were probably dumped in the Oder River with full awareness of the risks and consequences," he said in a video on Facebook. "We will not let this matter go. We will not rest until the guilty are severely punished."
German media have reported that the poison is mercury, although this has not been officially confirmed.