STORY: Worrying scenes from Poland's Oder river, where tons of dead fish have been hauled out of the water.
The Oder is Poland's second longest river flowing along part of the country's border with Germany.
Volunteers and anglers have removed at least 11 tons of dead fish.
Officials are warning people not to enter the water, or eat the fish, while the Polish government has pledged to investigate possible contamination.
Robert Suwad is head of the Polish Association of Fishermen in Wroclaw.
He says he's never seen anything like it and blames water contamination:
"Most likely, someone threw some toxic substances to the Oder River in a place that is beyond the borders of our activity (as the local fishermen association) and it has spread for a couple of kilometers along the Oder River and the cases of dead fish were spreading along with the flow of the toxic river."
Further upstream, on the river's banks in Germany, locals are noticing dead fish too.
Experts say low river levels due to the drought in Europe might have aggravated any possible contamination.
German freshwater expert Christian Wolter from the Leibniz Institute for Freshwater Ecology and Inland Fisheries isn't surprised by the mounting number of dead fish.
He says there could be many reasons for them:
"The fish are under stress and what exactly came in on top, a toxic substance, organic matter or construction work, dredging, the turbidity, anything can be the straw that breaks the camel's back."
Poland's environmental authorities have notified prosecutors about potential contamination of the river.
And the country's Prime Minister has vowed to punish those responsible.
One official called the situation a gigantic ecological catastrophe.