To be honest IMO looks like it could have been a shell, the initial flash may be lift charge igniting which is followed by the burst. Although most fireworks consist of a lift followed by a burst.......
The reason i say that is you hear of jo public getting their hands on things like shells and all to often they have no idea about mortars and how devices work, what else do you do if your not sure other than light the fuse and throw it on the floor and run?
An arrest has been made and the charges are to do with causing an explosion and using false documents. It doesn't say if the "false papers" charge is related to the explosives, though.