A German-Polish fight over a large pollution incident that killed thousands of fish in the Oder River is turning toxic.

The two countries were initially meant to unveil a joint report Friday analyzing what happened this summer on the river that in part flows along their mutual border.

Instead, Warsaw and Berlin each issued their own studies — further souring already tense Polish-German relations. In non-fish issues, Poland wants restitution for Germany's bloody occupation of Poland during the war and resents Germany's perceived reluctance to send more weapons to Ukraine, while Germany is worried at the erosion of rule of law in Poland.

“There were already a lot of conflicts between the Polish and the German side and what happened at the Oder this summer didn’t improve things,” said Hannah Neumann, a German Green MEP who traveled to the Oder in August.

Both reports blamed prymnesium parvum, a type of golden algae whose bloom can emit toxins lethal to fish and shellfish but is not harmful to humans. But they failed to agree on how much fault rests with humans.

German Environment Minister Steffi Lemke on Friday stressed that the “severe environmental disaster” was “caused by human activity.”

According to the German report: “The most probable cause of the fish kill in the Oder River is a sudden increase in salinity, which, together with other factors, led to a massive increase in a brackish water alga that is toxic to fish.”

Because the cause is thought to have originated on the Polish section of the river, German experts had to rely on data from Poland, but due to a "lack of available information” they said they could not determine what caused the “unnaturally high salinity.”

Poland didn’t provide Germany with all necessary information, and “the Polish side is annoyed about what they consider as finger-pointing from the German side," said Neumann.

The authors of the Polish report named a combination of natural and human factors, and argued they were unsure of the amount of polluted water discharged into the river. Under Polish law, water discharges can be legal under conditions set by so-called water permits.

“An analysis of the permits is underway but so far we haven’t found any problems with them from a legal standpoint,” said Andrzej Szweda-Lewandowski, head of the Polish General Directorate of Environmental Protection at a presentation of the report's main findings on Thursday. The full report is due to be released late Friday.

Polish NGOs hit out at that assessment. ​​“It was surprising to hear that the state has no problem with discharges of polluted water into the Oder as long as they are in line with water permits. It looked as if the Polish people were just told that the river was polluted fully legally,” said Piotr Nieznański from WWF’s Polish office.

In a report published this week, NGO Greenpeace blamed the fish kill on salt discharges from the Polish mining industry, based on the results of water and soil samples taken by activists at the end of August. Earlier this month, several major Polish scientific organizations — including the Polish Academy of Sciences — said the disaster was caused by the combination of environmental neglect and worsening conditions due to climate change.

The two countries are also at loggerheads over expansion plans for the river meant to allow for more barge traffic.

Unlike Poland, Germany has not yet started to deepen or widen the river. Lemke on Friday reiterated her concerns about the plans, arguing that the “expansion measures on the Oder River stand in the way of successful regeneration," and said she wants to work with her Polish counterpart Anna Moskwa "to agree on joint next steps.”

Poland insists that Germany should help carry out the 2015 plan to expand infrastructure on the river.

“The modernization of the Oder via construction of weirs and a container terminal is our priority. No step back,” Deputy Infrastructure Minister Marek Gróbarczyk said in late August.

This week's tit-for-tat reports aren't helping to figure out how to fix the river.

The failure to produce a joint report proves that Poland’s ruling Law and Justice party “is not able to overcome its aversion to our western neighbors and does not care about finding [a] solution,” said Łukasz Kohut, a Polish MEP with the Socialists & Democrats.

The current relationship does not seem “like a trusting, reasonable collaboration for the best possible future of the river," said Neumann.