Mexico’s president vowed on Saturday to redouble his fight against an epidemic of fuel theft after thieves punctured a pipeline north of Mexico City, causing an explosion that killed at least 73 people and injured 75 others. The blast underscored the deadly perils of the fuel-theft racket, which has cost the government billions of dollars a year and has been the target of a weeks' long crackdown by the administration of Mexico’s new president, Andrés Manuel López Obrador.