The FBI has investigated dozens of terrorist plots since 9/11. This is the first deadly bombing on U.S. soil that has succeeded since then.
Whether domestic or international, lone wolf or group organized, this attack caught the agency and Homeland Security off-guard.
There is little use speculating on who's to blame for the marathon carnage.
Investigators say all options are on the table and while no one has claimed credit for the attack typical when it's terrorism, the list of suspects is endless.
"This could be anything from a psychopath to a person with a grudge, to person seeking attention by mass murder, who says, 'look I did this by not using a gun,'" said Nicholas Casale, a Counterterrorism Expert.
Investigators are looking at the bombs, the two that exploded and, more importantly, the two that didn't.
The intact devices are being dismantled and analyzed for finger prints.
The bomb explosions are providing clues as well.
The white smoke points to black powder as the explosive material, the lack of structural damage to nearby buildings suggests smaller devices much less powerful than military IED's used in Afghanistan.
"The bomb has to be sophisticated because it went off on demand, however the construction of the bomb could be crude or simplistic because they didn't have the components necessary," Casale said.
The FBI says there was no threat prior to the race.
And while the attack has shades of Al Qaeda, it's far too early to say for sure.
Meanwhile, this former head of counterterrorism for the MTA says his gut is that this will be solved quickly.
"We're going to find out what the real motive was, and when we find out real motive we'll be surprised and don't think it's a complex motive as we think," Casale said.
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