“In the course of searches, evidence was secured showing that the suspect had gathered materials documenting, among other things, rail transports of military equipment,” Siemoniak wrote on Saturday on the X social media platform.
The court ordered the temporary detention on Saturday, Siemoniak said, adding that the man faces charges of "declaring readiness to act on behalf of a foreign intelligence service."
Siemoniak's spokesman, Jacek Dobrzyński, said that due to ongoing operational and investigative activities, no further details could be disclosed at this stage.
Poland has in recent years intensified counterintelligence efforts amid heightened security concerns linked to Russia’s war in neighbouring Ukraine.
Separately, the ABW has since March 2023 been investigating a case involving so-called “camera operators” accused of attempting to monitor trains carrying weapons and humanitarian aid to Ukraine, allegedly with the intent to derail them.
That investigation, overseen by the National Prosecutor’s Office in the eastern city of Lublin, focuses on the activities of a suspected organized criminal group acting on behalf of Russian intelligence services.
Authorities say the group, made up of at least 30 people, carried out espionage, sabotage and propaganda activities in Poland. In 2023, the ABW arrested 16 members of the group — 12 Ukrainian citizens, three Belarusians and one Russian — all of whom were later convicted.
Earlier this month, the ABW said it had issued a wanted notice for a Russian national suspected of organizing the group.
Investigators allege that the man, identified as Mikhail Mirogrodsky, acted on behalf of Russia’s Federal Security Service (FSB) by assigning espionage tasks, providing operational instructions and managing Telegram accounts used for communication within the group.