This was not your usual ‘fireworks display’ set.
This was a MORTAR CANNON. I have used them on several Independence Day Celebrations. The kit contains a plastic based cardboard tube and tennis ball sized ‘rounds’ with a long fuse. You can also buy extra ‘rounds’.
I wondered why these things had an EXTREMELY long fuse, until I lit the first one , stepped back about 10 feet, and watched it launch. I couldn’t hear for several minutes.
It occurs to me that not only may they have used these as the ‘improvised hand grenades’ that they threw at police, but (since they bought $4000 worth), just used the others in the bombs, as they were. No need to tear them open to get the explosive mixture out. Already have fuses. Most of the work is done. Just load them into a pressure cooker surrounded by nails and shotgun pellets. One or two ‘rounds’ would certainly do the job.
Link this to the reports of a nearby town having problems with someone shooting off some kind of ‘fireworks’ in the month prior to the bombing.
CORRECTION:
I meant to say, since they bought $200 worth. Someone else said $4000 worth and I repeated it, even though I had heard elsewhere it was $200.