The World Health Organization said on Saturday it still cannot independently confirm what caused the recent death of a 28-year-old Russian woman who worked at a lab studying plague and other diseases.

Darya Shipilova’s Oct. 2 death prompted the quarantine and medical observation of nearly 200 Russian citizens two days after she was hospitalized while working at the Irkutsk Anti-Plague Research Institute in Siberia. The initial laboratory results of “all identified contact persons” were negative, and none showed signs of illness, according to Rospotrebnadzor, the nation’s public health regulator. An initial, unconfirmed report claimed Shipilova became sick after accidentally breaking a vial in the laboratory.

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