Russian President Vladimir Putin routinely boasts that his country is leading the world in developing hypersonic weapons, which travel at more than five times the speed of sound.

But a number of Russian physicists working on the physics underlying them have been charged with treason and imprisoned in recent years, in what rights advocates describe as an aggressive crackdown.

Most of those arrested are elderly, and three are already dead. One was taken from his hospital bed in the late stages of cancer and died soon afterwards.

Another is Vladislav Galkin, a 68-year-old scholar, whose home in Tomsk in southern Russia was searched in April 2023.

Armed guys in black masks arrived around 04:00, searching into cupboards and grabbing documents with mathematical formulae on them, a relative says.

Mr Galkin’s wife, Tatyana, claims she has told their grandsons – who used to play chess with him – that he’s on a business trip. She alleges Russia’s security service, the FSB, has prevented her from speaking about his situation.

Since 2015, 12 physicists have been detained who are all linked in some way with hypersonic technology or with institutes that work on it. They are all charged with high treason, which can involve providing state secrets to foreign countries.

Russian treason trials are held behind closed doors, so it’s not obvious exactly what they are accused of. The Kremlin has said simply that “the accusations are serious” and it can’t speak further since special services are involved.

But colleagues and defence lawyers argue the scientists weren’t involved in weapons development and that some of the cases are based on them openly interacting with foreign experts. And some think the FSB wants to create the appearance foreign spies are hunting military secrets.

Hypersonic refers to missiles that can fly at extremely high speeds and also reverse direction during flight, evading air defences.

Russia says it has employed two types in its attack on Ukraine — the Kinzhal, launched from an airplane, and the Zircon cruise missile.

However, Kyiv says its forces have shot down some Kinzhal missiles, raising questions about their capabilities. As the technology has been developed and deployed, the arrests have persisted.

Shortly after Mr Galkin’s detention in April 2023, he was remanded in court on the same day as another scientist, Valery Zvegintsev, with whom he had co-authored many studies.

The state-owned news channel Tass has cited a source stating Mr Zvegintsev’s detention may have been motivated by a paper published in an Iranian publication in 2021.

Mr Galkin and Mr Zvegintsev are both named on an essay about air intake devices for high-speed airplanes published by the magazine.

In summer 2022, the FSB arrested two colleagues from the same institute as Mr Zvegintsev – its director and the previous head of a laboratory for work on aerodynamics at high-speeds.

Employees from the Institute of Theoretical and Applied Mechanics (ITAM) drafted an open letter in support of their three detained colleagues.

Now removed from the institute’s website, it said they were known for “brilliant scientific results” and had “always remained faithful” to their country’s interests.

It said the work they had provided openly had been extensively vetted for prohibited information by ITAM’s expert commission – and none had been identified.

“Hypersonic is a topic you are now obliged to put people in jail for,” says Yevgeny Smirnov, a lawyer with First Division, a Russian human rights and legal organisation.

Mr Smirnov defended scientists and others accused of treason in court before he emigrated from Russia to Prague in 2021, fearing repercussions from his activities.

He says none of the dozen experts had anything to do with the defence sector, but were exploring scientific problems such as how metals deform at hypersonic speeds or the impact of turbulence.

“This is not about making a rocket, but about the study of physical processes,” he explains, and points out that findings may be exploited later by weapons makers.

The arrests had began a few years earlier with Vladimir Lapygin. Now 83, he was arrested in 2016 but released on parole four years later. He had worked for 46 years at the Russian space agency’s principal research institute, TsNIIMash.\

Lapygin was convicted over a software package for aerodynamic calculations that he delivered to a Chinese contact. He says he supplied a demo version as part of conversations about potentially selling the whole package on behalf of the institute.

But he claims the version he gave did not contain any secret information, merely an example that has been “repeatedly described in open publications”.

Lapygin told the BBC that individuals arrested ostensibly in relation with hypersonics “had nothing to do with” building weapons.

Another scientist seized was Dmitry Kolker, a specialist at the Institute of Laser Physics, also in Siberia, who was arrested in 2022 while he was in hospital with severe pancreatic cancer.

His family said the charges against him were based on lectures he had delivered in China, but that the topic had been vetted by the FSB and that an agent travelled with him.

Kolker died two days after his arrest, aged 54.

“There’s a conflict within the system,” says a colleague of one of the arrested scientists, who requested to remain nameless.

Scientists are still expected to publish globally and interact with foreign colleagues, “meanwhile, the FSB thinks contact with foreign scientists and writing for foreign journals is a betrayal of the Motherland”, they argue.

The ITAM scientists believe the same. “We just don’t understand how to continue doing our job,” their open letter said.

“What we are rewarded for today… tomorrow becomes the reason for criminal prosecution.”

They warn that scientists are scared to engage in particular fields of study, while promising young staff are leaving science.

The letter was an unusual display of public support. The other institutes where arrested scientists worked have not commented. Other examples are also thought to pertain to international collaboration.

An probe into two other scientists was tied to Hexafly, a European effort to construct a hypersonic civilian aircraft, according to the lawyer Mr Smirnov, who worked on the case. That project, now concluded, was led by the European Space Agency and began in 2012.

The agency told the BBC “all technical contributions and exchanges were agreed and foreseen” in a co-operation agreement between the Russian and European parties involved.

Both scientists were sentenced to 12 years in prison last year, though Russia’s Supreme Court has ordered a retrial of one of them.

Other arrests connected to a study of the aerodynamics as a space ship re-enters Earth’s atmosphere.

It was funded by a European Union project and administered by the von Karman Institute of Fluid Dynamics in Belgium.

FSB inspectors were concerned about a rounded cone form that looked like a warhead in research that one of the scientists, Viktor Kudryavtsev, provided to the von Karman Institute, according to his wife, Olga.

The institute said the scheme, which ran from 2011 to 2013, “very clearly excluded military research”. It says it “could not find any trace of disclosing secret information” by Kudryavtsev’s staff. Human rights groups see a pattern.

Mr Smirnov alleges that, in private talks, FSB officers had confirmed to him that cases for sharing hypersonic secrets were being opened “to satisfy the wishes of those higher up”.

He believes the FSB wants to give the idea that agents are pursuing Russian missile secrets “to flatter the ego” of Mr Putin.

The cases came amid a greater surge in treason cases.

Sergei Davidis, who oversees work helping Russian political prisoners at the Memorial human rights group, speaks of a “atmosphere of spy mania and isolationism”, especially since Russia’s full-scale invasion of Ukraine.

Speaking from Lithuania, where his organisation went after it was banned in Russia, Mr Davidis says he believes the FSB, desperate to prove it is delivering, “builds up its reporting statistics through the fabrication of cases”.

But he believes there may be other causes in the arrests of scientists, such as competition for state contracts, or even a Kremlin message of displeasure targeted at all scientists active in hypersonics.

Mr Smirnov believes the FSB occasionally provides more lenient punishments if suspects confess and blame others.

Kudryavtsev was given a plea deal under which he would accept culpability and lay the finger at someone else, according to his widow, Olga.

He refused. He died of lung cancer in 2021, aged 77, before his lawsuit got to trial.

Retired FSB General Alexander Mikhailov believes the FSB “must ensure the confidentiality” of military technology.

He argues “undoubtedly” that there must be “substantial grounds” for severe sentencing such as the 14-year prison term handed down in May to one of the three ITAM scientists, Anatoly Maslov.

Gen Mikhailov argues the present surge in treason trials is the outcome of the spread of liberties and democracy in the 1990s.

He claims this led to a change in mindset from Soviet times, when he says those with access to state secrets were “thoroughly vetted” and “understood the responsibility” of releasing them.

“Some people were talking too much and leaks appeared,” he continues.

As for Mr Galkin, it is now over a year since the disguised agents arrived. His family believes he spent the first three months in solitary confinement.

Tatyana, his wife, says she is able to speak to him by phone through a glass divider and has even pondered asking to be jailed too “because he just sits there, day after day”.

“I may ask them to place me in the same pre-trial detention centre. It would be easy enough – you just have to suspect someone of something.”