Russian authorities have quarantined a hospital and are monitoring contacts of an employee at a plague research institute in Siberia who contracted pneumonia of an “unknown” cause. Local state TV said the woman died last week.

Russian authorities said they found no evidence that her illness was caused by any of the pathogens she worked with. But the case has raised alarm in Russia after unconfirmed media reports suggested she might have died of pneumonic plague — a less common but severe form of plague that can spread from person to person.

The woman worked at the Irkutsk Anti-Plague Research Institute of Siberia and the Far East in Irkutsk, southeastern Siberia. A state TV station in Irkutsk reported Monday that she died last week in a hospital in Shelekhov, a town just outside Irkutsk.

The hospital has been placed under quarantine, Mayor Maxim Modin said Friday, no longer admitting or discharging patients, and its laboratory halted carrying out routine tests. The hospital’s outpatient clinic kept operating as usual, he said.

Anna Popova, head of Russia’s public health watchdog, Rospotrebnadzor, flew to Irkutsk and met with Gov. Igor Kobzev and other officials on Friday.

Kobzev said everyone who came into contact with the woman is under medical supervision. He didn’t identify the infection and called it a suspected case of “a particularly dangerous infection.”

Rospotrebnadzor was implementing “a full range of anti-epidemic measures,” he added.

Rospotrebnadzor said Sunday the woman was diagnosed with “pneumonia of unknown etiology,” or origin, and said “extensive” testing found “no microorganisms associated with the patient’s professional activities.”

The agency also said it didn’t uncover any incidents at the institute “involving pathogenic microorganisms.”

Two people who were in contact with the woman tested positive for a virus that causes a common cold and two others for COVID-19, the agency said, adding that others who were tested were all negative.

In Washington, President Donald Trump said he was watching the situation with concern and said the United States would be willing to help, though he said he had not spoken to Russian President Vladimir Putin about the situation.

“It’s a rough disease. You know, that’s a disease we used to be able to control, but somehow those microbes have gotten stronger and smarter. They’re like an army,” Trump told reporters outside the White House. “Even with great advancement, the germs have really become relatively stronger than they were, and things that used to work on pneumonia don’t work as well anymore.”

U.S. Secretary of State Marco Rubio said Monday he doesn’t think there is cause for alarm, but the U.S. is watching the case closely, aware that “it could spread quickly” despite the quarantine and other public health protocols.

Independent local news outlet Lyudi Baikala quoted sources it did not identify as saying the 28-year-old lab technician died from pneumonic plague and that several Irkutsk hospitals were placed under quarantine.

Asked Monday to comment on the case, Kremlin spokesman Dmitry Peskov said the public and media should follow official statements from Rospotrebnadzor, rather than “rumors or speculation.”

Rospotrebnadzor did not immediately respond to a request for comment.

The Irkutsk Aluminum Plant in Shelekhov required staff to wear masks as a precaution, its director Artyom Fominykh said Saturday on Telegram, calling for calm and adding: “Better be safe than sorry.”

The World Health Organization describes pneumonic plague as a less common but severe form of plague — a disease caused by Yersinia pestis, a bacterium usually found in small mammals and their fleas.

“In the event of a potential laboratory exposure, it would be normal to identify individuals who might have been exposed and to monitor them according to the assessed level of risk,” said Dr. Malick Gibani, clinical associate professor in bacterial vaccinology at the Imperial College London.

The scale of any monitoring program would depend on “the circumstances of the exposure and the number of people potentially affected,” Gibani said in a written comment.

Ian Jones, professor of virology at the University of Reading, said that “plague carries a huge history and is still present in many parts of the world, including Russia,” although because of modern living conditions and antibiotics, “its potential is therefore limited.”

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AP reporters Matthew Lee, Will Weissert and Collin Binkley in Washington contributed.