An explosion rocked the skyscraper that houses the headquarters of Mexican oil giant Pemex in Mexico City on Thursday, causing a number of injuries, the company said. \"What took place was an explosion in the B2 building of the administrative center. There are injuries and damage on the ground floor and mezzanine,\" Pemex said on Twitter, as television showed black smoke billowing from the tower. The newspaper Reforma said on its website that at least 22 people were injured. Television images showed people being carted away in stretchers and even office chairs. The company had announced earlier that it had evacuated the building due to a power failure.