A friend of the Munich gunman has been arrested over suspicions that he knew of the attack plans but failed to report them.
The 16-year-old Afghan youth may also have played a role in creating a Facebook post encouraging people to gather at the city's train station, police said.
"There is a suspicion that the 16-year-old is a possible tacit accomplice to (Friday's) attack," police said in a statement.
Attacker David Sonboly, 18, killed nine and wounded 27 others in a rampage through the city before taking his own life.
He apparently plotted the attack for a year and is thought to have created a Facebook post offering free meals to teenagers at the McDonalds on the day of the shooting.