Six people have been arrested in the Democratic Republic of Congo for allegedly dumping highly radioactive minerals into a river, officials say.
Katanga has one of the world's richest belts of copper and cobalt
The authorities had ordered the nearly 20 metric tons of toxic material to be disposed of at an old uranium mine.
But some of those charged with the safe disposal of the waste are reportedly among those who have been arrested.
Residents of the large town of Likasi nearby are being warned not to use the water from the contaminated Mura river.
Authorities are still trying to trace the remainder of the waste, and an exclusion zone has been created around the site in south-eastern Katanga province.
Authorities in the region seized the 19 tons of copper and cobalt ore, which was due to be exported to a Chinese firm, last month.
Ore mined in Katanga, one of the world's richest belts of copper and cobalt, contains traces of uranium, which DR Congo is currently banned from exporting.
Tests have shown that radiation levels near the area where the material was dumped were nearly 50 times the legal limit.
"The damage is enormous," DR Congo's Environment Minister Didace Pembe, who led experts to the area, told Reuters news agency.
"We have asked people not to use water from the river and the entire commission charged with disposing of the minerals is now under arrest."
After seizing the copper and cobalt ore last month, the authorities had ordered its disposal at a nearby abandoned uranium mine, a report by the mayor's office in Likasi says.
An inquiry was launched after it was suspected that the radioactive material had been dumped into the river.
"Nineteen tons would be a small mountain. We did not see that, all the minerals were not dumped," Mr Pembe said.
He said the full extent of the contamination would be made public after the findings of the experts, who include International Atomic Energy Agency officials, are given to Prime Minister Antoine Gizenga.
DR Congo's national mining company, Gecamines, has been asked to begin a clean-up of the dumping site.