More than ten tonnes of dead fish have been found since late July in the Odra, Poland's second largest river, which also runs through Germany.
The toxic spill has been described as an ecological disaster, and there are fears the river could be affected for years.
Maciej Wasik, deputy minister of internal affairs and administration, told a press conference on Friday that "no heavy metals were found in the 50 samples of water from the River Odra."
He said that crisis management services are already working in the provinces affected by the disaster "to remove the effects of the river contamination."
Wasik also announced a ban on access to the Odra in the Zachodniopomorskie, Lubuskie and Dolnoslaskie provinces. He specified that water sports, bathing, angling and watering animals in the river is prohibited.
He added that the police, along with the local environmental protection inspectorates would check all industrial plants located along the Odra "in order to find out as soon as possible who caused this state of affairs."
Just what has killed river life and how it got into the river remains a mystery.
Meanwhile, German Environment Minister Steffi Lemke on Friday warned of an ecological disaster.
"I am very shocked and concerned about the fish dying in the Odra," she told the German portal Redaktionsnetzwerk Deutschland (RND).
"An environmental catastrophe is in the offing," Lemke said.
"All sides are working flat out to find the reasons for this mass die-off and minimise potential further damage," she added while vowing to support the authorities of the state of Brandenburg, which has been affected by the Odra contamination. (PAP)