Well for one thing, it definitely was not North Korea despite many paranoid claims.
A) They would have declared it internationally.
B) They wouldn’t have used basic homemade bombs.
C) They would have targeted a government/military installation, not a marathon
D) They’d much rather attack South Korea. Until the US intervenes with their war with South Korea, North Korea has no interest in the US
This looks like the work of maybe 1 or 2 messed up guys that were already in the area as opposed to some invading internationals. (I won’t rule out Al Qaeda but i’m not quick to blame them since they are usually happy to claim responsibility and gloat about their attacks. They haven’t done that yet, AFAIK )
Theres no sign of any clear target other than random people which makes me uncertain if any organised hate groups or fanatical militias were involved.
The attackers specifically chose a target and date that would inflict the most pain (physically and emotionally) but it doesn’t look like there is any other motivation (IE: Racially, politically, religiously etc), the attack is too random to indicate an intention like that. Methinks the attack was purely about wanton destruction. These guys must be some disgruntled psychopaths that just wanted to watch the world burn.
Also the attack may have been planned for a while, judging from all the bombs found littered about. It could have been much bigger than what it was.
The fact that several bombs failed to detonate or were intercepted indicates poor organisation or a small group of offenders.