* Government says 650 died in 1984 explosion
* Survivors believe twice that number died
* President pledges thorough probe of latest blast
By Dave Graham
SAN JUAN IXHUATEPEC, Mexico, Feb 4 (Reuters) - As Mexico waits for news of what caused a deadly blast at the headquarters of state oil giant Pemex last week, survivors of the worst explosion in the company's history three decades ago still wonder what exactly happened then and what became of their friends.
On Nov. 19, 1984, a liquid petroleum gas (LPG) plant blew up in this town on the northern edge of Mexico City, consuming hundreds of people in a firestorm so powerful the sky could be seen glowing red at the other end of the capital.
The explosion at the Mexico City offices of Pemex last Thursday reawakened memories of the San Juan Ixhuatepec disaster, and stoked complaints that the government will be no more adept this time around at telling the public what happened.
President Enrique Pena Nieto has vowed to conduct a thorough investigation of the explosion that killed at least 36 people at Pemex's headquarters. It is the first major test of his administration, which began in December.
However, the experience of San Juan Ixhuatepec shows that Mexicans are not easily convinced by the official line.
The federal disaster agency says 650 people died, 2,500 were injured and over 25,000 lost their homes in San Juan Ixhuatepec when a gas leak sparked a series of explosions that incinerated residents instantly, generating searing heat felt kilometers (miles) away.
Time and again locals say the official figure is far too low, and many put the death toll at more than twice that.
"They never told us the truth then, and I doubt they will now," said retired laborer Juan Pedraza, 65, watching a game of soccer in a brightly colored park in the heart of San Juan Ixhuatepec, where dozens of houses and shanty homes once stood.
"The government only counted the ones who died at the plant and in hospital," said Jose Espinosa, who was thrown to the ground by the explosion, which many first thought was an earthquake. "It was actually somewhere between 1,500 and 2,000."
Among those to disappear almost without trace in the government's rush to bury victims were two households close to him, one of which was a family of five, said Espinosa, 82.
"I went into their house afterwards and found the foot of the little boy. That was all that was left," he said.
Many speculate that the damage caused by the blast, which produced a giant fireball with a temperature of 1,200 Celsius (2,200 Fahrenheit), was played down to reduce compensation claims and popular discontent.
"And the same thing is happening right now," said another resident, Guadalupe Ramirez, 79. "They're never going to say what really happened."
Ramirez was fortunate not to be at home at the time of the blast, but his house was exposed to such intense heat that all his clothes were burned to a crisp.
Lying in a valley floor bisected by a foul-smelling river bed filled with garbage, San Juan Ixhuatepec's crowded street markets, convenience stores and peddlers have been completely absorbed by the advance of Mexico City.
Huge gas installations still fill the dusty town, which is traversed by a hulking motorway flyover nearing completion and a railway line. On either side of the tracks there are parks where homes destroyed in the blaze and the LPG plant once stood.
Francisca and Gustavo Gomez, a married couple, remembered how a family of nine who used to run a market in the town center were never seen or heard from again after the inferno.
"They just dug holes and buried people where they lay," said Gustavo, 59, pointing at the park, to one side of which now stands a grocery shop called "La Explosion" (The Explosion).
Entire houses were destroyed when six spherical LPG storage tanks and dozens of cylindrical containers, commonly referred to in the town as "sausages", began to explode. Cylinders fired off in all directions, two flying more than 1 km (half a mile) across town.
Survivors like Espinosa received compensation for their losses. He said his only possession to survive the blast intact was an effigy of Jesus Christ he had nailed to a wall.
Official records of the government investigation state simply that the gas leak stemmed from a burst pipe caused by a faulty valve. It was not made clear why the valve malfunctioned.
Exactly 10 months after the blast, a massive earthquake struck Mexico City, killing thousands - though again, the figure is widely believed to have been understated by the authorities.
"What happened here was just like the earthquake in 1985," said Rolando Zamorano, 52, an electronic goods salesman who at the time of the quake was a journalist in the capital. "There were many more dead then than the government admitted to."
History has taught the population of San Juan Ixhuatepec to be cynical. Twelve years after the big explosion, 83,000 barrels of oil went up in flames at the Pemex facility in the town, killing four more people and causing 5,000 others to be evacuated.
That incident also left loose ends.
An official summary of the blaze by the federal disaster agency gave the cause as a rupture to a storage tank valve due to the use of "unspecified material." (Reporting by Dave Graham; Editing by Kieran Murray and David Brunnstrom)