Polish authorities said on Thursday that toxic algae were to blame for the mass death of fish in the Oder, ruling out industrial pollution as the cause.

The conclusions, presented at a press conference by scientists and government officials, come from a preliminary report due to be released on Friday, weeks after the environmental disaster broke out.

The report’s authors said nearly 250 tons of dead fish were recovered from the Oder River, which flows through Poland and Germany.

Various factors “led us to conclude that the fish kills were probably caused by the toxic effects of an algal bloom,” said Agnieszka Kolada of the Polish Institute for Environmental Protection.

The microalgae in question – known as Prymnesium parvum or golden algae – are widespread in estuaries and usually grow in brackish waters, mainly near the sea, and “have never been detected in Poland until now,” she told reporters.

She added that the water quality of the Oder has been poor “for years” and has only gotten worse this summer due to high temperatures and very low water levels, which may have favored algal blooms.

Some observers, including media and environmental organizations, suspected that a chemical spill was responsible for the disaster, but the Polish side dismissed this theory.

“The fish kills were not caused by heavy metals, pesticides, or petroleum substances,” said a presentation of the report.

According to the analyzes so far, “none of the controlled companies have discharged pollutants in excess of the legal limits” into the river, said government department head for environmental protection Andrzej Szweda-Lewandowski.

The amount of industrial outflow is “the same as in previous years,” he told reporters.

Fishermen in Poland began reporting dead fish in the Oder in late July, and a few days later in Germany they began washing up.

Poland’s government only reacted on August 12, triggering widespread criticism from local Polish authorities as well as in Germany.