There could be other theories also. This may not not be the only one. A double cross could be part of the whole story. The two bombers were just minor players and they got exposed. There were many people there that day with backpacks. The liberal media was trying hard to point attention to a white bomber. Maybe the real bombers got exposed and the fall guy got missed due to too exposure.
I think that is exactly what happened, IMO
Of note, the Chechens are/were part of the Caucasion background population making up the so-called Circassians who were brought in as children to man the Ottoman spatznez in the late middle ages. Sometimes they are referred to as Christians but the Chechens CONVERTED to islam about 1300 to gain Ottoman protection against the Turks.