It supposedly was made out of "10-12 kilograms (22-26 pounds) of what appeared to be the Spanish-made Goma 2 Eco explosive, which was used in the Madrid bombings and is widely used in mining operations..."

Recall that 200lbs. of Goma 2 was allegedly given to the Madrid bombers by a colleague or friend. The good news is that their supplies of it might be running low, although that might not mean much.