User blog comment:Godisme/Hear.Fear.Here/@comment-5306073-20120720131838/@comment-4484409-20120720203748

It being Grimmjow makes the most sense. He would already be there and don't forget that he was an Espada while the other 3 were only Fraccion. They were going up against a way more powerful opponent who is now weakened and could easily get sliced in half by Grimmjow who had all this time to get more powerful since he wasn't confirmed dead. The person who did it pointed their sword at Urahara who viewed that person as a threat. Isshin wouldn't be a threat because they're friends, Aizen would still be in the Soul Society whether he was imprisoned or not, same thing with the Royal Guard. Grimmjow is the only one that fits.