Just got permission to share something from a friend who is a retired OSHA inspector.

This was an industrial plant, producing a chemical product, so OSHA has a strong interest in this explosion. The Dallas Region VI response team was probably on site by sunrise. They could have been there earlier, but OSHA is not a first responder and tries to stay out of the way until the victims are evacuated and the site is under control.

I know a couple of local OSHA warhorses that were up late watching the CNN coverage. Da-a-a-a-mn! This is a fire marshal’s nightmare. [There are other fertilizer plants across the state. Without knowing what this one was making, it is difficult to speculate on what caused the explosion. It was definitely handling bulk quantities of anhydrous ammonia, which is a potential fire hazard, although it is very difficult to ignite. Such plants tend to be poorly maintained and without adequate process controls to prevent such catastrophic incidents. I have seen plants that made ammonium sulfate (a non-explosive material not to be confused with ammonium nitrate) that had ammonia storage tanks that were rusted by exposure to sulfuric acid vapor. I have also seen plants that made ammonia and also urea, which were in very good condition ]

I have found two videos of the site explosion - one from a parking lot and another from a car window by the property fence. In both of them, if you do frame by frame (start/stop with the pause button), you can see that there is an initial explosion to the left of the camera (downwind of the plant fire). It is followed very quickly by a large secondary explosion from the site of the plant fire, which knocks the witnesses on their keesters. The initial blast is off frame, so you can’t see exactly where it started. It is apparent because of the visible flash from the left of frame. One video shows it in the air, as though it were an ignited gas cloud (a possibility). The other one was apparently closer to it, because the flash effect literally washes out the screen just before the big one goes. There is a significant amount of dirt churned up by the secondary, making it evident that there was significant cratering.

[These primary flashes could be artifacts of flash hitting the optics of the camera, but I doubt it.]
Possible scenarios:

1. A large LP gas storage tank ruptured and the subsequent vapor cloud ignited, leading to the disastrous secondary.
2. The fire released gases and combustion products that finally reached an explosive concentration downwind of the fire and flashed back to the source.
3. A natural gas line was ruptured, releasing natural gas vapor into a significant size vapor cloud, which ignited.
[4. If the initial flash was just an artifact of the camera, the fire ate into stockpiled materials within the plant to cause the big explosion. ]