Jesus.
Small explosive, definitely deliberate. Contact explosive, not much fire in that blast. The person who did this is organized.
Heads are going to roll for this.
The hospital reports that the injuries caused by this blast have resulted in many amputations.
However the explosion wasn't massive, it didn't wipe out a whole block, or perhaps , two of the largest buildings in the world for example. I doubt we are looking at an organization, definitely not the Iranians or North Koreans. My bet would be on a homegrown Islamic extremist, or failing that an ultra right wing extremist perhaps fundamentalists protesting against the proposed gun control laws.
Regardless of who is responsible, I hope everyone gets themselves well as soon as possible.