It's unclear what caused the explosion. However, Pemex's Twitter account stated that there was an explosion in the basement of the administration building and that investigations into the source were underway.

Smoke was coming out from the building and several windows were shattered. Pemex has dealt with similar accidents before: 30 people died in a Pemex gas plant explosion in Northern Mexico last September.
Firefighters and paramedics remain on the scene and are still searching for survivors.
 
UPDATE (6:31 pm): Pemex issued a press note where they sent their condolences to the families of those deceased and injured. 14 people were killed and 80 injured.

"Experts from the Attorney General's Office are analyzing the causes of the accident. Therefore, any version [currently circulating] as to the cause is unfounded." 

Reach Executive Producer Max Meyer by email, or follow him on Twitter.

Reach Executive Producer Francesca Martens here. Follow her here.