The man who killed six people in Krakow's main square on Saturday had looked at websites about Norwegian extremist Anders Breivik, Polish justice minister Waldemar Zurek said. He also confirmed that a silencer was found in the attacker's car.
The traces are on the suspect's computer, which investigators have checked. In a radio interview, Zurek said items and symbols that may be linked to far-right ideology were found in the premises where he worked.
Breivik carried out the terror attacks in Norway in 2011, including the massacre on Utoya island.
The shooting in Krakow left 16 people wounded, including two police officers and the attacker himself. He died in hospital on Monday morning.
So far there is no sign that the man left a letter or message explaining his motives. Investigators are checking his contacts, whether he had accomplices and what ideological influences played a part in the crime.
Police found the bodies of the attacker's parents and sister at their family home in Ostrowiec Swietokrzyski.