The deadly explosion which ripped off the headquarters of state-run oil giant Pemex in the Mexico City was caused by a build-up of gas, said the government.

Attorney General Jesus Murillo said that the experts from Mexico, Spain, Britain, and the US found no evidence of explosives in the blast.

The blast at the 54-storey headquarters of Pemex (Petroleos Mexicanos) killed 37 people and injured dozens. The explosion took place on the lower floors of the Pemex Executive Tower, which houses several thousands of workers.

"We have been able to determine that the explosion was caused by an accumulation of gas in the basements of the building. This caused a defect in the structure of the floors, which first were pushed upward, and then fell, which was the main cause of death in the building," Murillo said during a press conference.

He added that the experts believe an electrical fault triggered the spark in turn detonating the leaking gas. The gas was thought to be Methane, he added.

Murillo said there was no much evidence to support the explosion was a bomb blast as it would generally wreak a crater.

However, the source of the gas is yet to be identified as it could have leaked from a number of ducts or from the sewer system.

The blast had sparked off speculations of a targeted attack however denied by the authorities. The latest finding is likely to add up more criticism on the oil giant's poor record of safety measures.

The company has announced that the operation at the headquarters will resume on 6 February.

The oil giant, a key contributor to Mexico's federal budget, has suffered a string of accidents at pipeline and refinery installations in recent years. The state-run company is the world's fourth largest producer of crude oil.