A teenager who went on a shooting rampage at a Munich shopping centre last month, killing nine people and then himself, paid 4,350 euro (£3,770) for his gun and ammunition, German prosecutors said.
Authorities on Tuesday arrested a 31-year-old unemployed salesman who says he sold the Glock 17 pistol to the 18-year-old gunman illegally over the dark web.
That connection surfaced as the suspected dealer was being investigated for other sales.
Frankfurt prosecutor Alexander Badle said the dealer told investigators he sold the Munich shooter the pistol and 100 bullets for 4,000 euro on May 20.
He sold him 350 more bullets for 350 euro on July 18, four days before the rampage.
Investigators have said the gunman was a victim of bullying who suffered from panic attacks and depression.