“The search for the causes of the fish die-off in the Oder still haven't been completed,” said Andreas Kuebler, the ministry spokesman. “So far we have several organic and inorganic substances that could be responsible.”

“It seems to be a cocktail of chemicals,” he told reporters in Berlin. “According to our information so far, none of these substances alone led to the fish die-off. It must still be assumed that this could be a multi-causal incident.”

Dead fish in the Oder were first noticed by fishermen in southwestern Poland in late July, but German authorities said they weren't officially notified by their Polish counterparts until the second week of August. Poland’s government is trying to track down those responsible. Ten tons of dead fish have been removed from the river.