Considering Russian internal security claims they were caught fleeing to the west, towards Ukraine and that Putin has now stated that there was an arrangement for a hole in the border to be opened for them, it definitely seems like Ukraine is involved.

The fact they tried to flee at all instead of continuing the terror campaign also is an indicator. These guys could have stuck around and killed a lot more people if they weren't so concerned with escaping and their lives. Most of the suicide type attackers terrorist groups like IS recruit are indoctrinated to believe in fighting to the death and in dying and are more than willing to do so to increase the spectacular nature of the attack.

Reminds me of that series of attacks in I believe the Netherlands that was believed to be Gladio which occurred, Brabant was the name, the police would always conveniently arrive a minute after they'd left. Highly, highly coordinated attacks with military precision and escape routes.

The fact they claimed they were paid also rings similarly to claims by a past Ukrainian assassin who targeted an author or other civilian but was caught. No reasonable person would accept $6000 US dollars to be a mass murderer against a country like Russia knowing they'd be hunted for the rest of their life. That's hardly enough money to retire on or provide security for even a year even in much of the global south to say nothing of the global north where that would get you 2-3 months rent. And religious extremists like ISIS don't need payment to carry out their deeds. But nationalists, self-hating Russians, etc groomed by Ukrainian intelligence who want to do something anyways but aren't so hardened as to not be shaken up after like that guy? Fits the bill pretty perfectly.