Complex sentence: Kashubians who were suspected to support the Polish cause, particularly those with higher education, were arrested and executed, the main place of executions being Piaśnica (Groß Plaßnitz), where about 12,000 were executed.
Split into simpler sentences: Kashubians who were suspected to support the Polish cause, particularly those with higher education, were arrested and executed. The main place of execution was Piaśnica (Groß Plaßnitz), where about 12,000 were executed.