If you want the answer for yourself, you need to demystify "terrorism", since that is the headline narrative from day one and to this day. When you grok what terrorism really is, the logical conclusion to who did the deed is to answer the question of who benefited, what motives, and look for evidence.

Terrorism is a method of rulership over publics, not a tactic of war. Unless you consider the rule of terrorist ideology to be one of "at war with those under rule", but that is different model of thinking. Either way, terrorism comes down to low grade rule by mean and vicious people, the kind you would hope never to elect. Indeed, one might conclude as I have that terrorism is rule by the unelect.

Terrorism is not a tactic of warfare, it is violent coercion. Even that much is obvious, for it coerces people to become defensive, scared, etc. Civilians are not the target of military for war. Never have been. Only pawns can be used to enact terrorism, for no individual has anything to gain by the act.

No military strategy has ever used nor teaches terrorist methodologies. The response by the USA shows precisely why it is foolish, for it gains nothing to the attacker: no territory gained, only an enemy provoked.

The accused actors had nothing to gain unless they were pawns for reward, but then that would mean they did not plan it, nor conceive of it from the base notion of coercion toward a benefit. Nor did the accused actors have a motive. The narrative already becomes convoluted beyond this point, so I pull out my Swiss Occum Razor and shave it back. That is when I realize how deep the ideological methodology of terror-ism must be.

Terrorism is not the act of rogues and individuals, but this is what you are wanted to believe. Terrorism is the plan and rule of those found at the conclusion of logical procedure. Who benefited, what motives, following clues...