Investigators Friday were working to establish a motive for a deadly gun rampage in the centre of Prague a day earlier amid swirling questions and nationwide mourning over the Czech Republic's worst mass shooting since the immediate aftermath of World War II. The gunfire at Charles University turned the historic centre of one of Europe's most beautiful cities into a scene of carnage, its festive mood pierced by screams and the din of sirens. Tourists ran for safety. while some students barricaded themselves in classrooms.