The United States is closely monitoring the death of a 28-year-old worker at a Russian anti-plague research institute, Secretary of State Marco Rubio said on Monday, calling it a cause for focus but not alarm.

The worker, named in local media as Daria Shipilova, died of pneumonia of unknown origin at the Irkutsk Anti-Plague Research Institute of Siberia. Russian media reported she shattered a test tube containing the pneumonic plague pathogen on September 25 and fell ill two days later. Russia has not officially confirmed a lab accident.

Russian health watchdog Rospotrebnadzor said no micro-organisms linked to her duties were found and the situation in Irkutsk was stable. Its chief Anna Popova flew to Irkutsk on Friday. Dozens of contacts are under medical observation, and Kazakhstan has increased vigilance at border crossings.

“We’re watching and monitoring it closely. I don’t think it’s cause for alarm, but it is cause for focus,” Rubio told reporters. The White House said it is aware and assessing the situation.