President Donald Trump said the United States does not believe the death of a Russian lab worker was linked to a bioweapon. He spoke to reporters in the Oval Office on Wednesday.

The worker was identified as 28-year-old Darya Shipilova. She worked at the Irkutsk Anti-Plague Research Institute in Siberia. She died last week after developing pneumonia of unknown origin.

Russian officials quarantined or placed about 200 contacts under medical observation. Russia’s health watchdog Rospotrebnadzor said no plague was found among the contacts. It said reports of a second sick employee were incorrect.

Trump said Russian officials had assured Washington the situation was under control. He compared the secrecy to China’s early response to COVID-19. He said he has a phone call set up with Vladimir Putin.

The World Health Organization said it still does not know what caused the death. WHO chief Tedros Adhanom Ghebreyesus said the agency is seeking more information from Moscow.