The Polish Internal Security Agency discovered equipment for encrypting messages at his house during the arrest, Polish media reported.
The arrest took place in spring 2009 but was only made public on January 6.
News of his detention was known only by the president's office, the prime minister, and the prosecutor's office.
The unnamed suspect has reportedly lived in Poland for 10 years, speaks fluent Polish, and had a business selling accessories for hunting rifles.
He was reportedly under longstanding surveillance by the Polish security service.
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