A few months after Russia began its full-scale invasion of Ukraine in 2022, satellite imagery captured unusual activity at a restricted military research facility nestled among the birch forests northeast of Moscow.

The Russian site, called Sergiev Posad-6, had been quiet for decades, but it had a notorious Cold War past: It had once been a major research center for biological weapons, with a history of experiments with the viruses that cause smallpox, Ebola and hemorrhagic fevers.

Satellite imagery over the next two years — collected by Google Earth and commercial imaging firms MAXAR and Planet Labs — shows construction vehicles renovating the old Soviet-era laboratory and breaking ground on 10 new buildings, totaling more than 250,000 square feet, with several of them bearing hallmarks of biological labs designed to handle extremely dangerous pathogens.

There has been no sign such weapons have been used in the Ukraine conflict, but the construction of new labs at Sergiev Posad-6 is being closely watched by U.S. intelligence agencies and bioweapons experts amid worries about Moscow’s intentions as the conflict grinds through its third year.

The images showed multiple signatures that, when combined, indicate a high-containment biological facility: dozens of rooftop air handling units, layouts consistent with partitioned labs, underground infrastructure, heightened security features and what appears to be a power plant.

In recent weeks, Russian officials have publicly confirmed that the scientists will use the labs to study deadly microbes such as the Ebola viruses, in an effort to strengthen the country’s defenses against bioterrorism as well as future pandemics.

Under Centers for Disease Control guidelines, U.S. research on Ebola is restricted to laboratories rated as “biological safety level 4” (BSL-4), equipped to handle the most lethal and incurable kinds of viruses and bacteria.

The Russian Ministry of Defense did not respond to a request for comment. Here are the key findings, based on an analysis of satellite images and interviews with current and former U.S. intelligence officials — as well as military, biodefense and satellite imagery experts:

Four buildings have an unusually large amount of air-handling equipment, at a scale typically associated with high-containment laboratories.