MEXICO CITY — A mysterious explosion at the headquarters of Mexico’s state-owned oil company here on Thursday killed at least 25 people and injured 101, according to government officials, as windows shattered, the ground shook and thousands of employees fled into a panicked downtown. The cause of the explosion was not immediately known. It occurred just before 4 p.m. in the basement of an administrative building next to the 52-story tower of Petroleos Mexicanos, or Pemex. Company officials said there was significant damage to the first floor and mezzanine of the building, and witnesses said they saw rescue workers helping trapped employees who had been pinned under falling debris, while others dragged out the injured and the dead. Officials said the dead included 17 women and 8 men. “I saw them take out three people covered in blood,” said Trinidad Díaz, 31, the owner of a restaurant a block from the explosion. “And after that, ambulances started arriving, one after the other.”