Following a 23 March 1944 bomb attack in the Via Rasella by Italian resistance fighters that killed 33 soldiers, Hass, Capt. Erich Priebke and fellow officers rounded up 335 Italians and the next day transported them to the Ardeatine caves at the outskirts of Rome. Hass, Priebke, and their soldiers systematically executed each captive with a shot to the back of the head. The Ardeatine massacre is one of the most notorious in the Italian history of World War II.