No detail is too small for officials as they piece together what happened.
Investigators are making some headway. They now know the bombs were made out of pressure cookers and they discovered a circuit board in the debris.
But so far, there is no big break and no evidence that would narrow down who's behind the attack: a radical Islamist or a homegrown radical.
From a tip about possible explosives in a suburban Boston apartment, to the questioning of a Saudi student hospitalized with blast injuries, hundreds of federal agents have been deployed to track down any and every lead.
"I would say it's still totally up for grabs on who's responsible," said Samuel Rascoff, Terrorism Intelligence Specialist.
The former head of intelligence analysis for the NYPD says crudeness of the bombs suggests domestic terrorist but the synchronized explosions and the added shrapnel point to radical Islamists.
"Jihadi groups have been known to pack their bombs with lots of shrapnel to achieve maximum terror through maximum injury," Rascoff said.
"They even speculate it might be a particular group, but of course, all speculation," said Ahmed al-Rahim, an expert on radical Islam.
A former senior advisor on radical Islam for the U.S. Government has been scouring Jihadi websites for any possible clues. He's found repeated references to the timing of the attack; just two weeks before the anniversary of bin Laden's death.
"We have to be very aware about the period before the assassination of bin Laden that is an important date for jihadist and certainly al Qaeda," al-Rahim said.
Boston has the markings of the al-Qaeda inspired Times Square bombing attempt of 2010, yet also shades of the '96 Atlanta Olympic anti-government bomber.
Investigators know that this first bombing since 9/11 is terrorism, but what kind, who, and how many?
"In coming days, we will learn that this is the responsibility of some amount of coordinated effort some kind of group participation, the key question and open question, what kind of group and why would anyone want to do this?" Rascoff said.
Much has been made about the fact no group has claimed responsibility for the attack, a signature of organized terrorism.
But as one analyst pointed out, that could be a new strategy to avoid U.S. retribution.
Not to be overlooked is that this act of terrorism was carried out on Patriots Day, which celebrates the beginning of the American Revolution.
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