Police launched an operation earlier this month following a complaint from the Arsenal plant in the central town of Kazanlak about missing products and documents.

The missing papers contained "information which is extremely sensitive for the company and of interest to Bulgarian and foreign competitors," the ministry said.

The probe found that the three foreign employees, who worked on innovative technologies at the plant, had not returned to work after unpaid leave on September 17.

They had travelled abroad but returned to Bulgaria and visited Kazanlak on October 2, the day before the plant established the data had gone missing, the ministry said.

The two men -- a 24-year-old Lithuanian and a 60-year-old Russian -- and a 59-year-old Lithuanian woman were subsequently detained while trying to leave Bulgaria through the southwestern Kulata border checkpoint with Greece on Wednesday.

In their cars, police found papers and objects that they presume could be linked to the investigation. The three were arrested and were brought to Kazanlak for questioning on Thursday.

Arsenal is Bulgaria's oldest and biggest arms and munitions plant, which was the only licenced producer of Russian Kalashnikov assault rifles outside the Soviet Union during the Cold War.

After the fall of communism, the licences have been a constant bone of contention between Sofia and Moscow, which has repeatedly accused Bulgaria of counterfeiting the famous AK-47.

Arsenal was fully privatised in 2011 and is still making a wide range of assault rifles, heavy and light machine guns and ammunition.