An apparent wave of pollution has killed thousands of fish in southwestern Poland, according to a Thursday press statement from the Polish Waters ministry. The trouble seemed to start in a town called Oława but has spread along the rover Oder to Frankfurt, Germany more than 200 miles away.

“We are dealing with a gigantic and outrageous ecological catastrophe,” head of Polish Waters Przemyslaw Daca said in a news conference, according to a report from the Associated Press.

In Frankfurt, where the Oder flows along the border of the two countries, officials reported a major pulse of pollution in the river earlier this week. “Analysis results consistently show that on August 8th, 2022, a strong wave of organic substances passed through Frankfurt and has since continued downstream,” said a Thursday news release from theGerman state Brandenburg’s environmental ministry.

People living along the river have been told to not swim, fish, or even touch the water, according to another report from the Associated Press. Thus far, the cause of the alarming, ongoing disaster is a mystery.

One local Polish news broadcaster reported that high levels of mercury had been found in the river, based on state laboratory tests. However, the country’s water authority later said that mercury levels were still unconfirmed, according to a report from Reuters.

“The cause of the fish deaths on the Oder remains unclear even after the first test results are available. It is becoming apparent that a still unknown, highly toxic substance is flowing through the Oder,” said the recent German statement. The source and identity of the deadly substance are still mysterious, according to both German and Polish officials.

“We are most likely dealing with a crime where a substance was introduced into the water that causes the death of fish and other organisms,” said Poland’s deputy climate and environment minister Jacek Ozdoba.

Environmental authorities said they are continuing to investigate. The result of further water and chemical analysis are expected sometime today, reported the German ministry, which has also ordered the analysis of the dead fish.

Initially, communication between the two countries didn’t go as planned, hampering reporting and the speed of investigation, according to German officials. “It must be said that the reporting chains between the Polish and German sides did not work in this case...We have therefore contacted the Polish authorities and are asking for clarification immediately,” said Axel Vogel, the German environment minister.

Though not the main cause, drought in the region is believed to be worsening the river’s condition and the extent of the fish die-offs. With less water flowing in Europe’s rivers overall, any toxins that find their way into rivers and streams are more concentrated.

From the AP:

Piotr Nieznanski, the conservation policy director at WWF Poland, said it appears that a toxic chemical was released into the water by an industry and the low water levels caused by the drought has made conditions far more dangerous for the fish.

Europe’s ongoing drought could be its worst in 500 years, and climate change is likely a big contributing factor. Hotter temperatures mean more evaporation, less sustained snowpack, more agricultural demand on the supply, and thirstier plants. Climate change is also thought to be weakening the atmospheric forces that usually bring rain and snow to the continent, Peter Hoffmann, a meteorologist at the Potsdam institute for Climate Impact Research in Germany, told NPR.