A gas build-up caused the explosion that rocked the headquarters of Mexico's state-owned oil firm last week, killing 37 people, officials said Monday, ruling out a bomb attack. The announcement came four days after the tragedy at Pemex, a delay that triggered much speculation about the cause of the blast at the firm's offices in the heart of Mexico City. The explosion tore through a human resources building next to the company's skyscraper last Thursday afternoon, destroying lower floors, causing thousands of workers to flee into the street and injuring more than 120 people.