Authorities said the suspect had tried to buy a firearm and a hand grenade online and make explosives.
Hamburg’s top security official, Andy Grote, described the incident as “very, very serious”.
The 20-year-old German-Moroccan suspect — who has not been named — was taken into custody in August.
A judge ordered him to be kept in detention on suspicion of breaking firearms laws and indications that he planned to carry out an attack, Hamburg police said.
Officers searching his parents’ apartment following his arrest found extremist propaganda videos and bomb-making instructions.
A separate search at a cousin’s home elsewhere in Hamburg last month uncovered substances used to make explosives and several hundred metal nuts and screws.
RHM/PR