LUANDA, Angola – First the river turned red. Then tons of dead fish swam to the surface. Then thousands of people got sick.

Now 12 people have died in the Democratic Republic of the Congo in what researchers have called “an unprecedented environmental and human disaster” along the Kasai River, a southern tributary of the mighty Congo River.

Researchers and officials from the Congolese government say the cause was a toxic leak upstream of Angola’s largest diamond mine, operated by Catoca, a joint venture between Endiama, the Angolan state-owned mining company and Russian mining giant Alrosa.

The company admitted in a statement last month that there was a leak from its facility, but said it was just water and sand – nothing toxic.

In addition to the 12 fatalities, around 4,500 people became ill with diarrhea as a result of pollution, affecting nearly a million in total, Eve Bazaiba, the Congolese minister for the environment and sustainable development, said in a press conference Thursday.

“It is a total destruction of ecosystems, especially aquatic biodiversity,” said Ms. Bazaiba, who had traveled to the region.

She said that around July 26th, the people who lived near the water noticed that something strange was happening on the Tshikapa River, which flows north from Angola, where it spells Chicapa, then flows into the Congo and flows into the Kasai.

At first they thought small diamond miners were causing the problem, she said. But then, on July 31, the situation worsened.

“They noticed that there were dead fish. Lots of dead fish – tons and tons of them are floating on the river, ”said Ms. Bazaiba.

A team dispatched to the area reported that two hippos had also died. “Everyone panicked,” she said.

The government warned people not to eat the fish and took water samples to be tested in laboratories in Kinshasa, capital of the Congo. The results came back a week later. The water sample contained heavy metals – nickel and iron – and the pH was incorrect, according to the minister.

“It’s practically sour,” she said. “It sucks the oxygen out of the water. There is no more life. “

Researchers at Kinshasa University’s Congo Basin Water Resources Research Center described the pollution of the Kasai River basin as “an unprecedented environmental and human disaster.” In a report released in mid-August, they said they had tracked the spill from its source since July 15 in Angola’s Lunda Norte and Lunda Sul provinces, and it took 15 days to see the city of Tshikapa at the confluence of the Tshikapa and Kasai rivers . Two million people are at risk, it said.

The immediate aftermath of the disaster, the report says, has included water pollution, poisoning and loss of aquatic fauna and flora, water-borne diseases for local communities, disruption of fishing and shipping activities and lack of access to domestic water services.

It warned that pollution could spread downstream to the stretch of river that runs through the vast metropolis of Kinshasa, one of the most populous in Africa.

Ms. Bazaiba said she hoped the voluminous waters of the Congo – second largest after that of the Amazon – would dilute pollution when it reached the capital, adding that the water is gradually becoming clearer.

The government is now trying to determine the source of the pollution, she said, but must act because it came from a foreign country.

“We don’t know exactly whether it was an accident,” she said, “or whether it was known.”

Ms. Bazaiba said the Angolan government and the company recognized that the pollution was from the Catoca mine. She added that the Congo will seek compensation on the “polluter pays” principle.

But the Angolan government has not commented publicly on this issue. An official from the Ministry of Environment, Tourism and Culture, who was not allowed to speak publicly and spoke on condition of anonymity, said the ministry had not received any official information from the government of the Congo. The official said that the only information the ministry had through the media and the investigation was still ongoing.

An employee of the company, who was not authorized to comment on the matter and who spoke on condition of anonymity, denied that Catoca had confirmed the Congolese government’s allegation that a poison leak had occurred.

The Catoca mine produces three quarters of Angola’s diamonds. One of its owners, the Russian company Alrosa, has tried in recent years to increase sales in the USA.

In a statement last month, the company admitted that there had been a “break in the pipeline that functions as an overflow”. But it was said that only a mixture of sand and water had entered the river. A survey was carried out and “the recorded situation does not pose a threat to the life of the population”.

Catoca did not use the heavy metals described by the Congolese minister, said the company employee.

“No toxic materials may come from the Catoca mine because the mine does not use such materials,” said the employee. “It was a build-up of sand and water, or to be clear, it was mud.”