“It is now apparent that there was direct contact with someone who had a relevant influence on this attack,” said Bavarian interior minister Joachim Herrmann on Wednesday.
The communication appears to have been via an internet chat.
But Herrmann could not say whether this person was from terror group Isis, adding that this was currently the focus of investigations.
Investigators are also trying to find out where the chat partner is based.
The conversation took place intensively right up to the point of the attack, said Herrmann. But he added that it was unknown how long the chat had been going on for; if it had been over a period of weeks, months or longer.
The bomber, Mohammed D., killed himself in the incident in the small town in northern Bavaria. Fifteen other people were injured, four of them seriously.