A man in western Congo has died with hemorrhagic fever symptoms. Health authorities say that has led them to suspect that a still-unidentified virus may be involved alongside malaria in a mysterious outbreak that has killed dozens of people.

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.