Police looking for clues turned over every stone and flowerpot in Krakow’s eerily empty Main Market Square on Sunday as Poland entered two days of mourning for six people mowed down by a gunman with suspected far-right sympathies. Poland’s deadliest gun rampage in decades — in which a British man and an Austrian woman were among the victims — has left the country feeling vulnerable to the mass shootings that have spread across Europe in recent years. Dozens of police scoured the square the day after the frenzied killings in which the alleged 24-year-old gunman with a semi-automatic rifle drove around firing...