An initially "unknown" illness affecting hundreds in the Democratic Republic of the Congo may be attributable to malaria, malnutrition and a viral infection. But investigations are ongoing.

Congo’s health ministry told Reuters this week that malaria was the culprit: “The mystery has finally been solved. It’s a case of severe malaria in the form of a respiratory illness,” it said Tuesday in a statement, according to Reuters. The ministry did not respond to multiple inquiries from NBC News.

A mystery illness found in a remote part of Congo has been officially identified as a severe form of malaria, which has sickened nearly 600 people and killed 143.