The 28-year-old female researcher, who worked at the Irkutsk Anti-Plague Research Institute of Siberia and the Far East, died last week from “pneumonia of unknown etiology,” according to Russia’s consumer welfare service.

The agency said the “sanitary-epidemiological situation” in the Irkutsk region and the cities of Irkutsk and Shelekhov remained stable.

Russian authorities said they had found no evidence that the researcher’s illness was caused by any of the pathogens she worked with.

Reports citing a regional leader said nearly 200 people who had been in contact with the researcher were being monitored.

Maksim Modin, mayor of the Shelekhov municipal district near the border with Mongolia, said the inpatient facility where the researcher was treated had been closed for quarantine.

The Russian authorities have not confirmed the exact cause of the researcher’s death.