More than 10 tons of fish have been found dead in a European river since late July — an unexplained "ecological catastrophe" that is baffling scientists and political leaders alike.
Germany and Poland are working together to solve the mystery behind the mass die-off in the river Oder, which flows between the two countries.
Devastating pictures of the stricken fish were photographed from the Oder last week, as Europe sweltered in a widespread and destructive heat wave.
“Our border river Oder has been hit hard. The massive fish kill is frightening,” Axel Vogel, agriculture minister for the German state of Brandenburg, said in a German government statement Monday.
“We have to fear that the Oder will take a long time to recover from this catastrophe. This is also very bitter for Germany’s only flood-plain national park, the Lower Oder Valley.”
The Oder, known as Odra in Polish and Czech, runs from Czechia to the Polish-German border before flowing into the Baltic Sea. It is one of the biggest rivers in Poland.
The continent is suffering what experts say could be its worst drought in 500 years, with major rivers from the Danube to the Rhine drying up. And conditions are set to worsen in the weeks and months ahead, they add.
While climate change is not thought to be a main cause of the fish washing up dead in the Oder since July, experts have said that low levels of oxygen in the river — a result of unprecedented low water levels due to the heat wave — could have played a role.
“We are dealing with a gigantic and outrageous ecological catastrophe,” then-Polish Waters head Przemyslaw Daca said in a news conference Friday.
The main cause is thought to be water contamination due to a toxic substance, though officials have been unable to identify what it is.
People have been warned not to swim in the river nor to eat any fish from it. Polish soldiers and army reservists were called in over the weekend to assist a huge cleanup operation, Polish Defense Minister Mariusz Błaszczak said in a tweet.
So far removed more than 10 tons of dead fish.
Local and national politicians from Germany and Poland met Sunday and agreed to form a bilateral task force, the German government's statement said. Leaders from both nations said they suspect human-made pollution is to blame.
"The top priority is protecting the population, limiting damage and identifying the polluter," German Federal Environment Minister Steffi Lemke said at a news conference following the meeting Sunday.
"Experts are working flat out on the analysis in order to find out as quickly as possible which substances are causes of the mass death of fish and the damage to the Oder."
She added that by Sunday, at least 150 samples of water from the Oder River had been tested, but none of the studies had confirmed the presence of toxic substances.
"At the same time, we are testing fish. No mercury or other heavy metals have been found in them," Poland's environment minister Anna Moskwa said at the same news conference.
She said some Oder water samples were being sent to foreign laboratories to be tested for another 300 substances, including the presence of pesticides. She said on Tuesday that samples had been sent to the Czech Republic, Netherlands and the United Kingdom.
Polish Prime Minister Mateusz Morawiecki, whose right-wing government is under huge pressure to solve the mystery and stop more fish from dying, said Friday that “huge amounts of chemical waste” were probably to blame, adding in a Facebook video that “we will not rest until the guilty are severely punished.”
On Friday, Morawiecki said he had fired Daca Przemysław, head of the national inland waters agency, and Michał Mistrzak, head of the general environmental inspectorate, a government agency that investigates environmental disasters, because of their handling of the disaster.
Huge numbers of dead fish were first spotted near the southwestern Polish town of Olawa in late July.
German officials have complained that Poland failed to honor an international treaty by not notifying them immediately about the possible contamination of the river.
A boat captain first alerted German authorities about the dead fish on Aug. 9, Germany's federal government said.
“We know that the chain of reporting that’s envisaged for such cases didn’t work,” Christopher Stolzenberg, a spokesperson for Germany’s Federal Environment Ministry, told reporters in Berlin, according to the Associated Press.
NBC News has contacted the Polish Ministry of Climate and Environment for comment regarding the accusation.