I think the suspects the Russians arrested are probably innocent. They're Tajiks, who often migrate to the Moscow area for work. They were probably arrested because they were Muslims and foreigners. It is a Stalinist tradition, often repeated by his successors, to create "show trials." These show trials are where innocent arrestees are tortured beforehand to sign a confession, put on trial and publicly confess to being a part of an imaginary conspiracy. This was done in the 1930s, and has been revived by Putin, although on a much smaller scale.
I think the suspects in the attack may be put on trial publicly, where after torture behind the scenes, they will confess to being part of some kind of fabricated conspiracy with Ukraine and ISIS. There could even be Western conspirators in the imaginary mix, or other farfetched scenarios.
All my opinion.