Ljubljana - The lab that has performed chemical analysis of water samples taken after the fish die-off in Mali Graben, a river canal of the Gradaščica river in Ljubljana, could not confirm the presence of toxic substances in the water although they believe this is what happened.

"We believe that there have been multiple discharges of unknown substances - chemicals, cleaning agents, liquid manure or similar wastewater," Dušan Žigon, the head of the chemical division of the environmental laboratory at the Jožef Stefan Institute (IJS), told TV Slovenija on Thursday.

The contaminated water may have drained away by the time the dead fish were spotted and samples were taken.

The IJS does not perform analyses of samples from dead fish and even if it did the sample was not suitable due to the high temperatures, TV Slovenija reported in the news.

The report said though that the IJS did ascertain that the oxygen content in the water was sufficient for aquatic organisms, so the lack of oxygen due to low water levels and high temperatures could not have been the cause of such a large die-off.

Žigon estimated the damage caused at several ten thousand euros and said remedying the river could take a long while. The police continue with their inquiries, looking into potential indications of crime.

As much as 1.5 tonnes of fish died in the Mali Graben canal on Saturday, all specimens of all fish species. The local angling club described the incident as a horrendous ecological disaster. "The damage is irremediable, but if we had to asses it it would go into 100,000 euros and more," the Barje Angling Club said on Tuesday.

Ljubljana Mayor Zoran Janković called the incident a tragedy. He believes fines should be hefty if it turned the discharge was intentional.