There had been reports of a gas leak on the site prior to the explosion and they were trying to find the source of the leak and then the explosion happened.
There were people ACTIVELY looking for the source of the leak right before it happened. It took place in an extremely heavily fortified room with mechanical equipment which may have helped limit the extent of the injuries.
http://photos.signonsandiego.com/gallery1.5/ (photos here)
Photo 22, in the lower right hand corner, there’s two water heaters side by side, the right one appears severely damaged.