WARSAW, Poland — Laboratory tests following a mass die-off of fish in the Oder River detected high levels of salinity but no mercury poisoning its waters, Poland’s environment minister said Saturday as the mystery continued as to what killed tons of fish in Central Europe.

Anna Moskwa, the minister of climate and environment, said analyses of river samples taken in both Poland and Germany revealed elevated salt levels. Comprehensive toxicology studies are still underway in Poland, she said.

She said Poland’s state veterinary authority tested seven species of the dead fish and ruled out mercury as the cause of the die-off but was still waiting for results of other substances. She said test results from Germany had also not shown a high presence of mercury.

The Oder River runs from Czechia to the border between Poland and Germany before flowing into the Baltic Sea. Some German media had suggested that the river have been be poisoned with mercury.

Polish Prime Minister Mateusz Morawiecki said Friday that “huge amounts of chemical waste” were probably dumped intentionally into his country’s second-longest river, causing environmental damage so severe it would take years for the waterway to recover.

On Saturday, Morawiecki vowed to do everything possible to limit the environmental devastation. Poland’s interior minister said a reward of 1 million zlotys ($220,000) would be paid to anyone who helps track down those responsible for polluting the river.

Authorities in the northeastern German state of Mecklenburg-Western Pomerania warned people not to fish or use water from the Szczecin lagoon, as the river’s contaminated water was expected to reach the estuary area on Saturday evening.

“The extent of the fish die-off is shocking. This is a blow to the Oder as a waterway of great ecological value, from which it will presumably not recover for a long time,” said Alex Vogel, the environment minister for Germany’s Brandenburg state, along which the river runs.

The head of Polish waters, Poland’s national water management authority, said Thursday that 10 tons of dead fish had been removed from the river. Hundreds of volunteers were working to help collect dead fish along the German side.

German laboratories said they detected “atypical” levels of “salts” that could be linked to the die-off but wouldn’t fully explain them on their own.

Morawiecki acknowledged that some Polish officials were “sluggish” in reacting after huge numbers of dead fish were seen floating and washing ashore, and said two of them were dismissed.

“For me, however, the most important thing is to deal with this ecological disaster as soon as possible, because nature is our common heritage,” Morawiecki said.

His comments were echoed by Schwedt Mayor Annekathrin Hoppe, whose German town is located next to the Lower Oder Valley National Park. She called the contamination of the river “an environmental catastrophe of unprecedented scale” for the region.