The 16-year-old Afghan, arrested at his home in Munich, is being investigated because he is suspected to act as a possible accomplice, Xinhua quoted local police as saying.
Investigators also need to figure out to which extent the arrested person is responsible for a Facebook call for a meeting in a cinema complex near the Munich central train station.
On early Sunday evening, more than thousands of residents spontaneously took part in different memorial services.
The shooter, an 18-year-old German-Iranian teenager, started planning the attack one year ago. He had been living in Munich for two years, CNN reported.
The motive for the shootings was unclear and a state of emergency was declared in Munich.
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