MEXICO CITY — Rescuers searched for survivors and authorities promised a thorough investigation after an office building blast killed 25 people and injured 101 at the headquarters of Mexico's state-owned oil company, Petroleos Mexicanos.

The cause of the basement explosion in an administrative building next to the iconic, 51-story Pemex tower in Mexico City remained a mystery early Friday, with President Enrique Pena Nieto urging people not to speculate. Theories ranged from an electrical fire to an air conditioning problem to a possible attack.

"We have no conclusive report on the reason," Pena Nieto told reporters. "We will work to get to the bottom of the investigation to find out, first, what happened ... and if there are people responsible in this case, that we apply the full weight of the law against them."