Poland began two days of national mourning on Sunday after a gunman killed six people in Krakow’s historic main square, an attack the government said may have been inspired by far-right ideology.

Saturday’s shooting shocked a country that had prided itself on being safer than many of its Western European neighbors, and deepened public concern about extreme views following school stabbings in October that officials have linked to inflammatory content online.

Nationalist opposition politicians, however, questioned the grounds on which the government said far-right ideology could have led to the shooting that left 16 people wounded in addition to those killed.