Three Russian scientists who worked on hypersonic missile technology have been arrested on suspicion of high treason, sparking a rare public outcry from their colleagues and raising questions about the state of Russia’s weapons research.

The scientists — Anatoly Maslov, Alexander Shiplyuk and Valery Zvegintsev — were employees of the Khristianovich Institute of Theoretical and Applied Mechanics in the Siberian city of Novosibirsk. They were all detained over the past year, according to an open letter published on the institute’s website.

Russian Hypersonic Missile Scientists Arrested on Treason Charges

The letter professes the men’s innocence and praises their academic achievements, adding that all three chose to stay in Russia over highly paid and prestigious work abroad. “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,” it said.

The institute is known for its research on hypersonic technologies, which involve speeds of more than five times the speed of sound. Russia has claimed to have developed several hypersonic weapons, such as the Kinzhal air-launched ballistic missile, which President Vladimir Putin has described as “invincible” and “unstoppable”.

However, Ukraine recently claimed to have shot down six Kinzhal missiles during a barrage of Russian attacks on Kyiv last week, using U.S.-supplied Patriot air defense systems. This has exposed an “apparent vulnerability” and an “embarrassment” for Russia, according to an intelligence update from Britain’s defense ministry.

The arrests of the scientists come amid a surge in treason cases in Russia, especially against those who work in sensitive fields or have contacts with foreign countries. Last month, Russia adopted a bill that increased the maximum sentence for treason to life in jail.

The Kremlin said Wednesday that the scientists face “very serious accusations” but declined to provide further details. Kremlin spokesman Dmitry Peskov said he had seen the open letter and that Russian security services were continuing to work on the case.