An official inquest says the explosion that killed 37 people in January at the headquarters of state oil firm Pemex, in Mexico City, was caused by a mix of methane gas and solvent vapours.
The gas build-up in the basement rose from the soil and combined with the vapour of solvents used in maintenance.
The initial spark was caused by an electrical or mechanical source, experts say.
They also blamed faulty building design for the basement's poor ventilation.
The methane is believed to have come up from the soil underneath.
"[The gas came] either from oil spills by the Huasteca Petroleum Company, which occupied the site until the 1930s, or from the gas storage warehouse from Mexico City's administration, a building which also was located there," the report reads.
The experts say the methane would not have built up in the basement if the building had had proper ventilation.
Several lower floors collapsed in the blast, injuring more than 100 people.
Many were trapped in rubble at the base of the 54-storey tower after the explosion, that happened as shifts were changing in the afternoon.
After six months of investigation, experts found objects which could have sparked the blast: a light cable extension, a lamp and a plug.
Pemex has experienced a number of fatal accidents in recent years.
Mexico's President Enrique Pena Nieto is expected to propose a shake-up of Pemex in his energy bill.
The reform is expected to be presented to the Mexican Congress next week.