There were two confession letters. One (that they think is fake) from supposed Antifa people claiming they did it because BVB isn't doing enough against neo-Nazis in the fan base (which I'd not be surprised if it was actually written by such neo-Nazis). An attack of that sort seems utterly out of character.

Then there's an IS-style confession letter (but one that doesn't fit how IS usually does thing, starting with the fact that they don't usually leave letters at the attack site).

They have two suspects, one of them has been arrested now, but it's not really known yet what their motivation was (assuming they did it).