So it seems the lunatic or lunatics who did this used a pressure cooker bomb with low order explosives, possibly black powder or similar.
When I saw the videos I mentioned to a friend that they didn't look like HE and it seems I was right. Apparently you can even make them with smokeless powder or sugar / KNO3. I imagine this will make it bloody hard to trace unless they have some food forensics on what was used to construct the bomb.