German security officials said Friday that they have a foiled a planned Islamist attack, after arresting a man in the northern city of Hamburg over the summer who had been trying to buy weapons and make explosives.

Hamburg's top security official, Andy Grote, described the incident as “very, very serious,” German news agency dpa reported.

Authorities said the 20-year-old German-Moroccan citizen, whose name wasn't released, was taken into custody in August after trying to buy a firearm and a hand grenade online. 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 Islamist 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.

Dpa reported that the man's father was known to authorities and once held a position of responsibility at Hamburg's al-Quds Mosque, where some of the plotters of the Sept. 11, 2001, attacks on the United States had met.