Three Russian scientists who worked on the development of hypersonic missiles face “very serious” criminal charges, the Kremlin said on Wednesday in a treason probe that has raised alarm among Russian researchers, Reuters reported.

Kremlin spokesman Dmytro Peskov said during a daily news conference that he was aware of the open letter signed by the Siberian researchers who defended the three men, but that the situation was a matter for Russian security services.

Anatoly Maslov’s colleagues Oleksandr Shiplyuk and Valery Zvegintsev said in a letter published on Monday that they are innocent of the charges brought against them and that their accusation threatens to seriously damage scientific research in Russia.

“We know each of them as patriots and decent people who are not capable of doing what the investigating authorities suspect,” the letter reads.

Russian President Vladimir Putin has praised Russia’s hypersonic missiles for years, saying they are “invincible” and that Russia is the world leader in such weapons.

But Ukraine’s armed forces said on Tuesday that they had shot down six Russian hypersonic missiles, which can fly at speeds of up to 12,250 km/h, in one night, a move immediately disputed by the military leadership in Moscow.

Russia accuses researchers of treason because of information presented at international conferences

Various reports about scientific conferences show that Maslov and two other Russian researchers arrested by the authorities often attended presentations related to hypersonic weapons technology.

In 2012, Maslov and Shiplyuk presented abroad, in France, the results of experiments conducted on a model of a hypersonic rocket. In 2013, all 3 researchers were among the authors of a scientific paper on hypersonic technologies.

In an open letter signed by their colleagues from ITAM, the Kristianovich Institute of Theoretical and Applied Mechanics in Novosibirsk, it is said that the materials presented by the researchers at international conferences were repeatedly checked for the absence of classified information.

The Russian scientists also claim that their arrest shows that “any article or report could lead to charges of treason.”

“In this situation, we fear not only for the fate of our colleagues. We simply can’t figure out how to continue our work,” the open letter states.

Russian scientists remembered their colleague who died in the basements of the FSB

The letter also refers to the case of Dmitry Kolker, another Siberian researcher who was arrested last year on suspicion of treason and taken to Moscow for questioning, despite suffering from pancreatic cancer.

Kolker, a laser specialist, died two days later.

In their letter, Russian scientists note that such cases have a negative impact on young researchers in Russia.

“Even now (not in the conditions of the war in Ukraine) the best students refuse to work for us, and our best young people leave science. A number of research directions, which are critically important for the basis of aerospace technologies of the future, are closed simply because employees are afraid to participate in such research,” they complain.

When asked about the letter on Wednesday, Dmytro Peskov replied that “we did see this call, but the Russian special services are working on it.”

“They are doing their job. These are very serious allegations,” he added.