U.S. regulators are investigating why Delta Air Lines failed to recover as quickly as other airlines from a global technology breakdown and whether Delta’s treatment of passengers stranded by canceled and delayed flights violated federal rules. Transportation Secretary Pete Buttigieg on Tuesday announced the investigation into Delta’s response to the outage, which he said has affected more than half a million of the airline’s passengers. A Delta official said the airline expects to return to normal operations by the end of the week. Delta and its Delta Connection partners canceled more than 500 flights Tuesday.