Russian investigators have arrested an award-winning 74-year-old space scientist, who is suspected of passing missile secrets to a Nato country.
Viktor Kudryavtsev is in custody in Moscow, the state space agency Roskosmos confirmed.
On Friday security agents raided a Roskosmos research facility called TsNIIMash in Korolyov, near Moscow.
The alleged spying concerns hypersonic missiles, which fly at more than five times the speed of sound (Mach 5).
On Friday Russia's Kommersant daily reported that about 10 staff at TsNIIMash were under suspicion. A director's office was among the areas searched there.
Suspects could be charged with high treason, which carries a maximum prison sentence of 20 years.
The Federal Security Service (FSB) also searched offices at the United Rocket and Space Corporation (ORKK) in Moscow.
On Thursday the Russia defence ministry released video of two new hypersonic missile systems - called Kinzhal and Avangard. Both can deliver nuclear warheads.
Mr Kudryavtsev is an expert on the dynamics of gases and liquids and won a state award for his research.
In 2016 he and a group of scientists signed a letter in support of a fellow TsNIIMash researcher and spacecraft designer, Vladimir Lapygin, who was convicted of treason.