The Centers for Disease Control and Prevention said Tuesday there is currently "no indication of a broader threat" to the United States following reports that a Russian laboratory worker may have died from pneumonic plague.

The case involves 28-year-old Daria Shipilova, who worked at the Irkutsk Anti-Plague Research Institute in Siberia. She died Oct. 2 at a hospital in Shelekhov, a city near Irkutsk, after reportedly becoming ill.

Russian media reports have attributed her death to pneumonic plague, but authorities have not officially confirmed plague as the cause of death. Russia's public health watchdog said Shipilova had been diagnosed with pneumonia of unknown cause and that testing did not detect microorganisms associated with her work at the anti-plague institute.

Conflicting reports have also emerged about how Shipilova may have become infected. One account said she accidentally broke a test tube containing plague bacteria at the laboratory, while another said she became sick after returning from a trip to Thailand. Russian officials have not confirmed either explanation.

Authorities placed nearly 200 people who had contact with Shipilova under medical observation. Meduza reported that regional officials said 189 people were hospitalized for observation, although there has been no confirmation of additional infections among the institute's employees.

The CDC said the U.S. government is coordinating with other federal agencies and monitoring developments. Pneumonic plague is a serious bacterial disease, but it can be treated with antibiotics when identified early. Unlike other forms of plague, pneumonic plague can spread from person to person through respiratory droplets during close contact.

The State Department similarly urged caution while warning against concluding unconfirmed reports. A department spokesperson said the U.S. government was monitoring the situation and encouraged Russian authorities to provide accurate information quickly and openly.

Russian officials have also sought to reassure the public. Irkutsk Oblast Gov. Igor Kobzev said Monday that no other employees at the research institute had sought medical treatment for infectious diseases and described the sanitary and epidemiological situation in the region as stable.

Secretary of State Marco Rubio said Monday that the reported case was "a cause for focus" rather than alarm. His comments came as he traveled to Reykjavik, Iceland, as part of a multistop European trip.

President Donald Trump also addressed the reports Monday, describing pneumonic plague as a "rough disease" and offering U.S. assistance. Trump said he had not yet discussed the case with Russian President Vladimir Putin.

The situation has drawn particular attention because Shipilova worked at a facility dedicated to researching plague and other dangerous infectious diseases. However, there is currently no official confirmation that the illness was caused by plague, that a laboratory accident occurred, or that the disease spread beyond those placed under observation.

Russia's last confirmed human case of plague occurred in 2016, when a 10-year-old boy in the Altai Republic contracted the disease after butchering a marmot.