A 28-year-old lab technician in Russia’s far east working on research at the country’s Anti-Plague Institute died of “pneumonia of unknown etiology,” on Thursday, triggering a public health lockdown involving the quarantining of 200 other people.

Daria Shipilova, 28, was hospitalized and put on a ventilator on Tuesday, then died two days later. The research facility is located within Irkutsk Oblast, in Siberia. The diagnosis of “pneumonia of unknown etiology” was given by Rospotrebnadzor, Russia’s public health and consumer protection agency, in the conclusion of its investigation by a special expert commission. Kremlin spokesman Dmitri Peskov urged the public to keep calm and only listen to professionals from the group regarding the situation, rather than “rumors and speculation.”