The Kremlin said on Wednesday that three Russian academics who have worked on hypersonic missile technology face “very serious accusations” as part of a treason probe that has alarmed the country’s scientific community.

Dmitry Peskov, a spokesperson for the Kremlin, said that while he was aware of an open letter from Siberian scientists defending the men, the security services should handle the situation.

Colleagues of Anatoly Maslov, Alexander Shiplyuk, and Valery Zvegintsev protested their innocence and said that the trials threatened to seriously harm Russian research in a letter that was released on Monday.

“We know each of them as a patriot and a decent person who is not capable of doing what the investigating authorities suspect them of,” they said.

Vladimir Putin has bragged that Russia is the world leader in hypersonic missiles, with the ability to escape opposing air defenses by traveling at speeds of up to Mach 10 (12,250 kph). Russia denied Ukraine’s claim on Tuesday that it had destroyed six of the weapons in a single night.

The accused scientists were regular attendees, according to notices of scholarly conferences that date back many years.

At a symposium in Tours, France, in 2012, Maslov and Shiplyuk presented the findings of an experiment on the development of hypersonic missiles. A book chapter titled “Hypersonic Short-Duration Facilities for Aerodynamic Research at ITAM, Russia” was written by all three of them in 2016.

The materials the scientists had presented in international forums, according to an open letter from their ITAM (Kristianovich Institute of Theoretical and Applied Mechanics) colleagues in Novosibirsk, had been thoroughly reviewed to ensure they did not include prohibited information.