In the dead of night, in a chaotic situation, is it credible that a skilled killer could find a way to hide/otherwise evade the fuzz? Hell yes, it is, and doubly so by these movies' standards.
Bottom line is, if Nolan had had our heroes successfully blame Dent's rampage on Joker and his henchmen, no one
in the audience would have cried foul, certainly not after the massive leaps in logic seen in the nearly two movies building up to that moment. The deaths were pinned on Batman purely for an "oh, snap!" cliffhanger ending.