Mexico City ( CNN ) -- Workers digging through the rubble of what were offices at Mexico 's state-run oil giant have found more victims , raising the death toll from last week 's blast to 35 . Pemex , the oil monopoly , on Sunday confirmed the figure , which was above the 33 reported killed earlier . More than 100 people were injured ; two people remain missing . Pemex , the powerful but much-criticized state oil company , has said it 's not clear what caused the blast , which took place Thursday at an administrative building in the Pemex office complex in Mexico City . Authorities have said investigations are under way to determine whether the explosion was the result of an accident , negligence , or an attack . `` People were screaming . ... You could see pieces of the wall falling to the ground , '' said Joaquin Borrell Valenzuela , an attorney for the Pemex comptroller 's office , who was in a courtyard outside the building at the time of the blast . Paramedics quickly arrived and started pulling people from the debris . Thousands of people work at the Pemex headquarters , which includes a 54-story building that is nearly 700 feet tall . The annex where the explosion happened is adjacent to the tower . Mexican oil giant chief wo n't speculate on cause of deadly blast . CNN 's Mauricio Torres contributed to this report .
At least four people were killed in last night 's explosion . Two people remain missing . The blast happened at the Mexico City headquarters of Pemex , the state - run oil giant . Operations are under way .