Russian scientists helped make their country a leader in developing cutting-edge missiles that fly at least five times as fast as the speed of sound. Then Russia started calling them traitors.

A Moscow court this week sentenced Alexander Shiplyuk, 57, the director of a Russian physics institute who specializes in hypersonic flight, to 15 years in prison for treason. Though the trial has been shrouded in secrecy, Mr. Shiplyuk’s advocates say he was accused of illegally sharing classified information. It was the latest step in a yearslong crackdown on some leading Russian physicists, a prong of the Russian government’s wide-ranging campaign of repression notable for its overlap with the country’s military industry.

Another senior scientist in Mr. Shiplyuk’s institute, the Khristianovich Institute of Theoretical and Applied Mechanics in Siberia, was sentenced to 14 year in prison in May, and a third was arrested last year. At least eight other physicists working in fields related to hypersonic and supersonic flight have been arrested since 2015, according to Perviy Otdel, a group of Russian lawyers who specialize in treason and espionage cases.

The cases against the scientists have been kept under wraps, with hearings held behind closed doors. Mr. Shiplyuk’s lawyer said on Wednesday that he could not comment on his client’s case because it was “secret information.”

But the scientists’ supporters say that the accusations against them stem from the sort of collaboration with colleagues abroad that is the lifeblood of their profession. They say that Russia’s F.S.B. security service, the main successor agency to the K.G.B., has waged a systematic campaign against Russian scientists who work in fields with sensitive military applications — a campaign that is likely to have a chilling effect on such research.

“We are not only afraid for the fate of our colleagues,” said an open letter from fellow scientists in defense of the Khristianovich institute’s scientists, published on its website in 2023 and later removed. “We simply do not understand how to continue doing our job.”