Thomas de Maiziere told reporters in Berlin that the shooter, a German-Iranian 18-year-old, had likely used a hacked account to “invite (the victims) to go to the branch, offering them special reductions”.
“Some of our findings suggest that the killer” broke into a girl’s account and used it to send invitations to go to the restaurant at 4:00 pm (1400 GMT) on Friday.
“I will give you whatever you want, for not a lot of money,” the online invite read, according to German media reports.
The shooter, David Ali Sonboly, killed nine people and injured 16 others at the Olympia mall in Munich.
He fled the shopping centre and then committed suicide, with his body found later Friday night a kilometre (0.6 miles) from the scene of the rampage.
Most of the victims in Friday’s attack were foreigners, including three Turkish nationals, three people from Kosovo and a Greek man.
The casualties were mostly young, with three aged just 14 and two aged 15.
De Maiziere said Sonboly had likely been bullied by other “young people his age”.