Prime Minister Mateusz Morawiecki, whose government is under pressure over its handling of what appears to be a major environmental disaster, has promised that Polish authorities will hold the perpetrators accountable.

“Huge amounts of chemical waste were probably dumped into the Oder 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 culprits are severely punished.

German media have reported that the poison is mercury, although this has not been officially confirmed.

Investigations are underway to determine the cause of the massive fish kills. A large number of dead fish were first spotted near the town of Olawa in southwestern Poland in late July, along with dead animals such as beavers.

Przemyslaw Daca, head of Polish waters, the national water management authority, said on Thursday that 10 tonnes of dead fish had been removed from the river.

“It shows that we are dealing with a gigantic and outrageous ecological disaster,” he said at a press conference near the river where officials faced angry residents.

Meanwhile, German officials have complained that Poland failed to honor an international treaty by not immediately informing them of the possible contamination of the river. A boat captain first alerted German authorities to the dead fish on August 9.

“We know that the reporting chain envisaged for such cases has not worked,” Christopher Stolzenberg, spokesman for Germany’s Federal Environment Ministry, told reporters in Berlin.

Stolzenberg said German authorities were in contact with their Polish counterparts for further information on the situation and to provide any requested assistance.

Poland has deployed soldiers to help clean up the Oder and a fishermen’s association in Zielona Gora, a town in western Poland, said on Friday it was suspending fishing in the river due to contamination.

According to Morawiecki, the scale of the pollution is so great that it may take years for the river ecosystem to recover.

Defense Minister Mariusz Blaszczak announced on Thursday that soldiers and reservists were being deployed to help remove pollutants from the river, known as the Oder in German and the Odra in Polish and Czech. It flows hundreds of kilometers north from the Oder Mountains in Czechia and empties into the Baltic Sea.

Frank Jordans in Berlin contributed.

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