That's what I thought at first-that it was Spanish dynamite stolen from a native mine. Then I heard it was titadine, a French dynamite popular with ETA. Now, if you believe El Mundo, it was an ANFO/RDX mixture. There's also some confusion about the origins and techniques used for the timers. What can I say, except the Spaniards themselves don't seem to be that confident about the official version.
This all looks like that Gen. Petain conspiracy jibberish about the OK City bombing. I think the info they had after the attack and the months following was correct. N. African jihadis did it with the stolen dynamite. There's nothing to this, but buzzwords and mystery.