Murillo described a "diffuse" blast that moved slowly and horizontally, typical of the detonation of a cloud of gas, rather than an explosion that would have emanated from a relatively compact source like a bomb.

The announcement late Monday ended days of a near-total lack of information about the potential cause of the incident. The sparse information spawned a torrent of complaints about government secrecy and speculation about the cause of the blast, most focusing on the possibility that it had been set intentionally.

Murillo said there is not yet any evidence of criminal wrongdoing in the disaster, but the possibility of criminal charges remained open.

The blast also generated debate about the state of Pemex, a vital source of government revenue that is suffering from decades of underinvestment and has been hit by a recent series of accidents.

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