German police have arrested the alleged administrator of an underground "dark net" website that was used to procure the firearms involved in a 2016 gun attack in a Munich shopping mall in which nine victims were killed.

Police arrested the 30-year-old suspect last Thursday, prosecutors said on Monday, after an investigation led them to the hidden discussion forum with 20,000 registered users where weapons and drugs were being traded in a "marketplace" section.

"The sale of the weapons used in the rampage in Munich on July 22, 2016 was ... carried out on this platform," prosecutors said. Attacker David Ali Sonboly, 18, shot himself dead after the rampage, during which a further 27 people were injured.

The arrest came amid mounting concern among authorities around Europe that such secretive marketplaces make it too easy for criminals and militants to obtain weaponry that has traditionally been highly regulated across Europe.

Police concluded the German-born Sonboly was a deranged lone gunman obsessed with mass killings who drew no inspiration from Islamist militancy.

According to the prosecutors, the weapons website was also used to trade forged money and identity documents as well as stolen credit card and bank account details.

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