Terrorists don't have access to make bombs like that. At least not without setting off a dozen red flags.
Doesn't look like anything contrived or too serious. I'll wait for the investigation and see what happened.
My first guess on this was a munitions depot, based on what looked like small ammo explosions before the big whump, but then ANFO came to mind. Just a whoops in storage protocol. (And I can guarantee you it was not nitrogen triiodide.)