Here's an update from Reuters:
Reuters Article$this->bbcode_second_pass_quote('', 'M')exico blast kills at least 33, flagging Pemex safety woes
<<snip>>A Pemex official said the damaged area was used for human resources in the corporate and refining divisions. It did not have a boiler or gas installations, the official said.
Former Pemex worker Ricardo Marin, 53, said there was nothing in the building which would explode and that the kitchen, where there would be gas, was on the other side.
"The only thing that occurs to me is that it was an attack - but against whom? There's no one with an important job down there," he said, waiting outside the Pemex hospital where a friend was in intensive care. "Maybe it could be a message to Pena Nieto, but not even that has any logic."
Pemex office worker Alfonso Caballero, who was one floor above the blast at the time, said he did not smell any gas and guessed it had been caused by machinery.
Mexican officials have not ruled out sabotage.
<<snip>>Whatever caused the explosion, the deaths and destruction will put the spotlight back on safety at Pemex, which only a couple of hours beforehand had issued a statement on Twitter saying it had managed to improve its record on accidents.
"I suspect this was a bomb," said David Shields, an independent Mexico City-based oil analyst. "There are clandestine armies across Mexico, not just the (drug) cartels."
Shields pointed to the bombing of several Pemex pipelines in the eastern state of Veracruz in 2007. A shadowy Marxist rebel movement took credit for some of the blasts.