MEXICO CITY - Rescuers dug through the night while investigators searched for answers on Friday after an explosion rocked the headquarters of Mexican state-run oil giant Pemex, killing 32 people.

Another 101 people were injured in the explosion, which sent shocked employees pouring out of the 54-floor Mexico City skyscraper beneath a pillar of black smoke, some carrying wounded people out on office chairs.

Hundreds of firefighters, police and soldiers toiled through the night after the blast ripped through an annex of the tower on Thursday, but the cause of the blast remained a mystery while officials cautioned against any speculation.

“A good part of the rubble was removed and we hope to be able to finish the removal in the morning,” a city civil protection spokesman told AFP, adding that the search focused in the basement. Survivors described an earthquake-like rumble that shook the floor and shattered windows after the explosion heavily damaged the annex’s ground floor and mezzanine. Witnesses said a roof connecting the annex to the tower collapsed. At least six ambulances were at the scene in case any people were found, while police partially reopened traffic on the heavily-travelled avenue in front of the complex. Officials said 46 people remained hospitalized. “We were waiting all night to assist in a major emergency that did not materialize because, fortunately, it appears that almost everybody was taken out,” a military nurse who refused to give her name told AFP. Floodlights shined on the rubble and two cranes were brought to help rescuers in hard hats and surgical masks look for survivors. One survivor was found almost six hours after the blast, which took place around 3:40 p.m. (2140 GMT).

Pemex director general Emilio Lozoya Austin was scheduled to hold a press conference early Friday.