(Bloomberg) -- Poland is seeking help from abroad to discover the source of toxic contamination in the Oder River following a fish die-off that environmental groups have described as the country’s biggest environmental disaster in years.

The government in Warsaw sent water samples to labs in the Czech Republic, the Netherlands and the UK, Climate Minister Anna Moskwa said on Twitter. Over the weekend, Polish emergency services pulled 80 tons of dead fish from the river -- Poland’s second longest -- and banned swimming and the use of boats.

The contamination sparked a diplomatic tiff with Germany, where local officials accused Poland of ignoring the disaster when it began more than two weeks ago.

Authorities in Germany’s Brandenburg region found traces of mercury in the water -- although not enough to trigger a massive fish die-off-- as well as high salinity and chlorophyll, along with elevated temperatures and unusual alkaline levels. Polish tests show no mercury or any other toxic substance so far.

European Environment Commissioner Virginijus Sinkevicius said on Twitter that he’d spoken with Poland’s climate minister and offered support for the investigation. A spokesman for the European Commission said Tuesday that the bloc doesn’t have “any definitive reason” for the death of the fish and is ready to assist, depending on the needs of Poland and Germany.

Polish authorities said on Tuesday evening that water parameters began to improve. They also saw number of dead fish started to drop. According to Climate Minister, labs continue to search for any toxins. They also examine whether the catastrophe may be caused by drop of large quantity of water used by industry that could have increased salinity, overlapping with low water levels and creating deadly conditions for fish.

The Oder runs from the Czech Republic to the border between Poland and Germany before flowing into the Baltic Sea via Szczecin Lagoon. Poland’s government has offered a 1-million zloty ($220,200) reward to anyone helping identify those responsible. Authorities also plan to increase penalties for people who cause environmental damage.

(Updates with Polish authorities comments in sixth paragraph)

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