The arms, believed to be owned by a private company and intended for delivery to Ukraine, were found stored in containers at a hangar in the village of Laszki, in southeastern Poland’s Podkarpackie province, according to officials.
The District Prosecutor's Office in the nearby city of Przemyśl said it opened the investigation on May 27 following a notification from Poland's Internal Security Agency (ABW).
"The investigation concerns the illegal possession and storage of weapons and ammunition by a private company in Laszki," said Marta Pętkowska, a spokeswoman for the prosecutor’s office.
She added that such an offence is punishable by up to two years in prison.
Pętkowska also said the weaponry was "being transported to a designated secure location."
Poland's defence ministry said in a post on X that the arms did not belong to the Polish military.
It added that "appropriate services" were "securing the site and equipment."
Interior ministry spokesman Jacek Dobrzyński told reporters that the weapons included anti-aircraft systems and that they were likely part of a shipment meant for Ukraine.
He called the lack of supervision over the facility a "scandal."
Poland has served as a key transit hub for Western military aid to Ukraine since Russia launched its full-scale invasion in 2022, the Reuters news agency reported.
Russia invaded Ukraine on February 24, 2022, starting the largest armed conflict in Europe since World War II.
Wednesday is day 1,197 of the war.
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Source: IAR, PAP, Reuters