The World Health Organization said on Saturday that no case of plague had been registered after a laboratory technician at a Siberian plague research institute died, and that it still did not know what caused the pneumonia that killed her.

Russia’s state health regulator, Rospotrebnadzor, told the agency that tests on the woman and on people who had been in contact with her were negative for the dangerous pathogens they were screened for, the WHO said in a statement, as Al Jazeera reported. Her contacts still have no symptoms. No other cases of severe pneumonia linked to her have been found.

“WHO has not identified evidence of ongoing transmission associated with this event,” the statement said. It also named what is missing: “A key uncertainty remains the absence of a confirmed diagnosis in the deceased laboratory worker.”

Russian media identified the woman as Darya Shipilova, 28, a technician at the Irkutsk Antiplague Research Institute of Siberia and the Far East. She died on Oct. 2, and about 200 people were put in quarantine. A local official said on Oct. 6 that 60 percent of them had been released.

Pneumonic plague, which is caused by the bacterium Yersinia pestis, was suspected early on. The governor of the Buryatia region said she had “died, maybe of the plague.” Rospotrebnadzor gave her diagnosis as “pneumonia of unknown aetiology.”

Russian officials said Ms. Shipilova had been properly vaccinated, that no microorganisms connected to her work were found in her body and that post-mortem tests didn’t detect plague.

The WHO’s director general, Tedros Adhanom Ghebreyesus, asked Russia on Wednesday about reports that a second employee at the institute had pneumonia. Moscow dismissed those reports as “false information.” On Thursday, Russia said it had finished health checks on everyone who had been in contact with the technician and had shared all of its information with the WHO.

The United States has also pressed Russia to be open about the case.

Earlier in the week, Russian officials said there was no risk of an epidemic, and a WHO spokesperson put the risk at moderate to low for the area where she died, low for Russia and very low for the European region as a whole.

Health officials have watched closely for unexplained deadly pneumonia since the Covid-19 pandemic. Even so, the cause of a single case of pneumonia often can’t be determined.