cue up this video to @2:04 and notice the rapid fire plume and white smoke combination. This is an indicator of an incendiary reaction. Very likely that the device was blown clear from the explosion site and ignited afterward.

Also notice the very suspicious body language of the man near the triage blanket in the foreground. He sees the plume and pauses, raising both hands to his head in an 'oh fuck' moment. He turns and begins to walk away. He turns once more, probably realizing he had to play his role. He looks around to see if anyone had noticed, and then he proceeds.