BERLIN—German police detained a 19-year-old Syrian man suspected of planning a terror attack and seized bomb-making material at his home, prosecutors said Tuesday, underscoring the threat posed by potential Islamist terrorists in Germany.

Federal ​prosecutors said the man, named as Yamen A., was arrested by armed police early Tuesday morning. The man had been in touch with Islamic State agents, ​prosecutors​ said, and it wasn’t clear if