Three people have died of “pneumonia of unknown cause” in Argentina, according to health officials.

As the health minister of the northwestern province of Tucumán, Luis Medina Ruiz, announced on Thursday, nine cases of the respiratory disease were found in a private clinic there. Eight of the sick are or were nurses at the clinic. Three of those infected have died since Monday.

The health authorities are now trying to find out which pathogen triggers the pneumonia. During the investigations in a renowned laboratory in the capital Buenos Aires, the corona virus, influenza A and B flu viruses and hantaviruses were already excluded as possible causes.

According to the information, the three deceased are two nurses and a 70-year-old patient who had been admitted to the clinic for an operation.

On Wednesday, the Minister of Health said that in addition to a pathogen, possible “toxic” and “environmental causes” would also be checked. The water and air conditioning systems would be examined.