Strzok, or an agent just as putrid, most likely murdered Seth.
My guess is that the union goons belonging to SEIU or someone off their back bench made the hit on SR, and then they got whacked to tie up loose ends.
Shot placement was kind of amateurish, so there is that angle. Pros normally double tap in the right spots.
DC Metro never released any info. from area cameras that might have had visual or acoustic clues.
Metro police chief resigned within months if not weeks of the shooting/coverup and was shortly thereafter hired to be head of NFL security.
Possibly Strzk, but more likely an MS-13 or DC thug paid by Strzk