It was either ISIS or AQ.
No one else uses suicide attacks against soft targets.
Even if it's some loon radicalized by ISIS via the internet it is an ISIS attack.
ISIS will claim responsibility for anything that instills fear in western countries.
The interesting thing will be if its an isis inspired attack or if this was directly planned by isis themselves.
It doesn't matter
that much. If you're assigning yourself to be "fighting on behalf" of a radical terrorist movement, you are part of it. Even if you haven't been spoken to directly from some terrorist sitting at "ISIS HQ". The message sent out from radicals is all people who align themselves with the cause and act out are one and the same.
These terrorists are largely homegrown. They haven't been born in war-torn countries and have a direct "bone to pick" with the Western countries they end up in. They are often born here and through indoctrination/reading/preaching and getting involved in fundamental circles, they radicalise and then act out, often for the cause of martyrdom. The trends being an overwhelming majority of males, which I've said a few times. However, the trends as of late also often being born nationals, rather than people coming into the country from abroad. However, sure, their families/parents or even a lot of the preachers and radicals in their circles may well have been born abroad. Sometimes though, they are born abroad and sickeningly use the migrant crisis as a way to get into countries.
Males being radicalised and the concept of martyrdom are two things that really need to be focused on. Like it or not in terms of doctrine, the concept of martyrdom itself is largely spread throughout the Islamic world. Becoming a warrior or fighter for the cause of God, and sacrificing one's self for a promise of reward. A lot of widespread terrorism uses guns and bombs to kill, but actual suicide bombing is often almost exclusively linked to one kind of radical terrorism.