North Korea, Al Qaeda, racists, or something else? I honestly think anyone could claim responsibility but given the high explosives and the seemingly professional nature of the attack I'm guessing either NK, AQ, or a false-flag to start a war on the Korean peninsula. edit: Nice downvotes? We cannot speculate who?
North Korea hasn't ever done anything like this so it's truly out of line to begin speculating that it's a "false-flag to start a war on the Korean peninsula" - way to blame the victim
Are you just trying to start an argument?