A Russian lab tech at the Irkutsk Anti-Plague Institute in Siberia died last week after contracting what might have been the most dangerous form of plague, pneumonic plague, after possible exposure to Yersinia pestis, the bacterium that causes the disease.

Conflicting reports in Russian media have only added to the fear and confusion. Alexey Tsydenov, head of the Buryatia region near Irkutsk, where Darya Shipilova contracted the infection, first said that she died of the plague before changing his post to say that she “possibly” died of it.

Rospotrebnadzor, Russia’s health watchdog, said the 28-year-old died from “pneumonia of unknown etiology.”

The lab where Shipilova worked was locked down, with people inside forced to sleep on chairs and the floor. Relatives are allowed to drop off food at a certain checkpoint. Several hospitals in the area are under similar quarantine. Testing has not revealed further cases of the infection, but 200 people remain under observation.

Secretary of State Marco Rubio said the United States is closely monitoring the situation, saying that travel is already limited between the two countries, but that nevertheless, once a pathogen like the plague escapes, it can travel quickly.

“There’s very limited travel between the U.S. and Russia, obviously, but like anything else, something like that gets out, it could quickly spread,” he said. “So we’re watching and monitoring it closely. I don’t think it’s cause for alarm, but it is cause for focus and a cause to just keep an eye on it. And we’re doing that.”

Pneumonic plague, unlike its more well-known cousin bubonic plague, is the most dangerous form of plague, affecting the lungs and traveling through respiratory droplets between people who are in close contact. It acts quickly, potentially killing its victims within 18 to 24 hours if not immediately treated with antibiotics. It takes four to seven days for symptoms to appear.