Complex sentence: After capture they were initially held in the prison at Grini where they were interrogated, before they were handed over to the Sicherheitsdienst (better known as the SD) and transported to Sachsenburg concentration camp in Germany.
Split into simpler sentences: After capture they were initially held in the prison at Grini where they were interrogated. Before they were handed over to the Sicherheitsdienst better known as the SD and transported to the Sachsenburg concentration camp in Germany.