The Polish government said on Tuesday that no toxic substances have been detected in samples of the Oder river where a mass die-off of fish took place, adding to the mystery over what happened in the waterway along the Polish-German border.
The announcement in Warsaw echoed the inconclusive findings released hours earlier in Germany.
Polish Environment Minister Anna Moskwa also said no evidence of pesticides had been found in the samples of dead fish, which have washed up in the last two weeks on the banks of the river.
Moskwa said there remains three working hypotheses for the ecological disaster and that the presence of harmful foreign substances has not been ruled out.
The first hypothesis is the possible entry of a toxin into the water, either during the production process at an industrial plant located on the Oder river or through an illegal discharge.
The second involves natural causes, including high temperatures and salinity, low water levels and increased concentrations of pollutants.
The third option being studied, she said, was the possibility of a discharge of a large amount of industrial water containing chlorine, which could trigger pollution in river bottom sediments.
On the other side of the border, the first tranche of results from tests on dead fish and water by a German state laboratory confirmed that they did not contain particularly high values of heavy metals such as mercury.
A spokesperson for the Brandenburg state Environment Ministry, Sebastian Arnold, said that experts could not yet say whether the mass fish die-off on the river had one single cause.
"We have detected high salt loads and a high oxygen content," he added.
The German state laboratory is examining further water samples from different days and various measuring points as well as more samples from fish carcasses, Arnold said.
The results of regular tests over the past 31 days on the agency's website shows that certain values in the river changed dramatically from August 7 onwards.
The oxygen content, the pH value, the turbidity and other values suddenly increased, while the amount of nitrate-nitrogen dropped significantly.
The fish deaths have led to warnings of an environmental catastrophe and recriminations between Germany and Poland as to why the problem was not spotted and reported sooner.
On Tuesday, Polish emergency services reported that they had recovered almost 100 tons of dead fish from the Oder and from the smaller Ner river.
The Ner is not connected to the Oder, but dead fish were also discovered there a few days ago, also without a known cause.
According to Polish authorities, dead fish have now also been found south of the port city of Szczecin in canals connected to the Oder.
This means that the contaminated water masses are moving towards Szczecin, said the head of the regional administration, Zbigniew Bogucki.
The European Commission said on Tuesday it stands ready to help in any investigation.
"We are prepared to help with any means possible in terms of expertise or exchange of information with other countries in order to find responses and possible solutions," a European Commission spokesperson said.
The commission is in contact with Warsaw to see "what Poland needs, what expertise can be provided," another spokesperson added.
The announcement follows talks between between the Polish Environment Minister Anna Moskwa and EU Environment Commissioner Virginijus Sinkevičius.
