Mexico oil firm blast caused by gas build-up

The explosion that killed 37 people at the headquarters of Mexico\'s state-owned oil firm was caused by a gas build-up, officials said, ruling out a bomb attack. The announcement ended a four-day-long mystery that had sparked a wave of speculation about the cause of the blast at the complex that houses the offices of Pemex in the heart of Mexico City. The explosion tore through an annex of the company\'s skyscraper in the capital last Thursday, injuring more than 120 people. \"We were able to determine that the explosion was caused by an accumulation of gas in the basement\" of the annex, Attorney General Jesus Murillo Karam told a news conference. The blast caused floors to collapse, killing 37 people, he said. \"We confirmed that there are no traces of explosives,\" he said, adding that the blast did not leave a crater and that the victims did not have the type of dismemberment caused by bombs.