[6] In the two months leading up to the incident, local plant safety committee inspections revealed that: 30–40% of safety devices (including firewater spray) were bypassed or non-operational; housekeeping was substandard; pressure gauges were in bad shape and inaccurate; a relief valve on an LPG-receiving manifold was missing; an additional relief valve was needed for the Minatitlán pipeline, after operational flowrate had been increased to 11,900 cubic meters/day (75,000 barrels/day).

[1] The settlement of San Juan Ixhuatepec long predated the disaster,[10] but housing surrounding the facility itself began to materialize only after the construction of the installation started in 1962,[9][11][12] although this is disputed.

[15] A flash fire ensued, which immediately transitioned to a violent vapor cloud explosion (VCE), likely due to its flame front acceleration being enhanced by the especially congested geometry of the plant.

[5][14] In a textbook case of the domino effect accident,[14] the explosion damaged further piping and storage tanks, which resulted in a massive conflagration fed by multiple LPG leaks.

[4] The explosion was witnessed by the pilot of a Pan Am flight on approach to the airport, who communicated to air traffic control that he believed a nuclear bomb may have exploded in the city.

[1][14] At the Gasomático site, 100 parked trucks loaded with LPG household cylinders weighing 20–40 kilograms (44–88 lb) were completely burned out and hundreds of secondary explosions took place.

[5][6] Initially, director-general of Pemex Mario Ramón Beteta and government officials attempted to shift the responsibility for the accident onto the gas-distributing businesses adjoining the terminal.

[13][22][23] No results from a public inquiry or a Pemex investigation have ever been made available, except for a declaration of the Attorney General[6] released on 22 December 1984, which pointed to the gas escape having occurred at the row of bullet tanks next to the spheres.

This investigation was not instigated by the Mexican authorities or Pemex, but rather was a scientific mission whose purpose was testing physical models used in safety studies for the prediction of damage from industrial explosions, and examining the emergency response to the accident.

Although the Dutch team did not primarily focus on the causes of the accident, the final report hypothesizes that the initial leak may have been caused by overpressure and rupture of a pipe to one of the cylindrical vessels (possibly due to the high discharge pressure (> 60 bars (870 psi)) of a booster pump in one of the underground pipelines), likely combined with a tank overfill and the consequent opening of a tank relief valve discharging to atmosphere.

[9] An informal investigation was conducted by the U.S. Department of Transportation, which corroborated the TNO's findings on the cause of the accident, additionally pointing out that the overpressure should have been prevented by relief valves placed on the receiving pipelines, but these had not been installed.

Their report also called attention to an earlier statement by Pemex that the pipelines were not isolated, with LPG being pumped into the terminal at the usual rate until 6:40 a.m., long after the onset of the accident.

[24] A team from KAMEDO (Katastrofmedicinska organisationskommittén, or Disaster Medicine Organization Committee) of the Swedish National Defence Research Institute were sent to Mexico for an ex-post evaluation of the medical emergency management.

[28] They found that the response in treating the burns of the injured from San Juanico had been effective and supported by considerable local medical resources and experience.

[28][29] President of Mexico Miguel de la Madrid and other high-ranking government officials drew criticism for not attending either the funeral ceremony or the mass burial of 272 of the victims.

The parish priest of San Juan Ixhuatepec may have been removed to elsewhere in the State of Mexico by church authorities, after complaining that much of the financial aid allocated to the affected families was quickly disappearing.

[22] The disaster was detrimental to the ruling Institutional Revolutionary Party (PRI), as – official reports notwithstanding[25] – inadequately maintained Pemex infrastructure was generally seen as responsible for the explosions, which were soon overshadowed by the 1985 Mexico City earthquake.

[10] For example, concerns were raised on the uncontrolled growth of the housing areas immediately adjoining the LPG terminal, which was against the Ley del petróleo (Petroleum Act), establishing minimum safety distances and mandating approvals from the Secretariat of Health and Assistance.

[22] Although the accident became well known worldwide in the technical domain of process safety, it had relatively little mainstream resonance outside of Mexico (e.g., as opposed to the Bhopal tragedy, which followed it by merely two weeks), likely due to the fact that no non-Mexican companies were involved.

In 1996 another Pemex site in San Juan Ixhuatepec suffered a serious accident, this time involving a petrol tank, which led to the death of a fireman and the evacuation of around 5000 people from their homes.