Polish prosecutors have questioned 228 witnesses in relation to the mass death of fish in the Oder river along the Polish-German border that continues to perplex officials.
Investigators, together with witnesses, also made 12 site visits to different sections of the river, Deputy Prosecutor General Krzysztof Sierak said on Wednesday.
Sierak said it was still unclear what caused the incident, which has been worrying local residents for days.
The Polish government assumes that a large amount of chemical waste was discharged into the river. The Polish police have offered a reward worth $213,000 for information on the culprit.
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 said they had recovered almost 100 tons of dead fish from the Oder and from the smaller Ner river.
In Germany, researchers are probing the role of a toxic algae species that developed rapidly in the river.
Christian Wolter, an expert from the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin, said on Wednesday that it had been identified as Prymnesium parvum.
"The species is known to occasionally cause fish deaths," he told dpa. However, he stressed it was still unclear whether the algae's toxin is the reason for the deaths.
Wolter spoke of a massive algae bloom of 200 micrograms per litre and more than 100,000 cells per millilitre of water. For humans, however, the toxin of the algae is harmless, he said.
Wolter said he did not personally believe the incident was an accident.