The state prosecutor said the two arrests were of people thought to know the suspects, rather than the suspects themselves.
Three men were shot in the ice cream parlour De Rocco after at least three men entered and started an argument.
The website of newsmagazine Der Spiegel reported that one of the shot men had owned a betting shop and the argument was about unpaid gambling winnings. But the conflict got out of control and ended in the wild shoot-out.
One man died immediately, another died in the ambulance while being taken from the scene and another remains seriously injured under police protection in hospital.
A woman, thought to be around 50, who was in the next door Greek restaurant, was hit in the stomach by a stray bullet and died at the scene.