An analysis of river water taken this week showed evidence of “synthetic chemical substances, very probably also with toxic effects for vertebrates,” the German state of Brandenburg’s environment ministry said on Thursday, adding that it remained unclear how the substance entered the water.

According to local German broadcaster rbb, the state laboratory found high levels of mercury in the water samples.

However, Wladyslaw Dajczak, the head of Poland’s Lubusz province, quoted by PAP news agency said that tests run on Aug. 10 and 11 showed mercury was found only in “trace amounts”, well within allowed levels.

He said a barrier would be set up on the Oder near the city of Kostrzyn to collect dead fish flowing down the river, with 150 Territorial Defence Forces soldiers delegated to help with the clean-up.

The head of Poland’s national water management authority said the situation was serious and that by Thursday evening Poland had collected over 11 tonnes of dead fish.

“In my opinion, there was chemical contamination on the Oder river by some industrial plant,” Przemyslaw Daca, the head of Polish Waters, was quoted as saying by Polish Radio 24.

“(It) is being investigated by the prosecutor’s office, the police and local environmental protection inspectorates. The problem is enormous, the wave of pollution runs from Wroclaw to Szczecin. Those are hundreds of kilometres of river, the pollution is gigantic.”

A spokesperson for the German environment minister told a news conference on Friday that they were following the situation closely, and that it was not yet clear what had got into the water.

“We have an incomplete picture,” the spokesperson said. “We need clarity on what materials are in the water.”

(Reporting by Anna Wlodarczak-Semczuk and Pawel Florkiewicz; Additional reporting by Thomas Escritt, Writing by Rachel More; Editing by Hugh Lawson, Mike Harrison and Toby Chopra)