There wasn't an instant of doubt about "When?" and "Where?" But a day after the Boston Marathon bombings, "Who?" and "Why?" remained frustrating questions with no clear answers.
Here's a look at what's not yet known about the explosions that killed three and wounded more than 170:
WHO? With no credible claim of responsibility, and no arrests of any suspects, authorities took pains Tuesday to stress that their investigation remained wide open. From President Barack Obama on down, they suggested that the bombings could be the work of a foreign or domestic group, or of an individual.
Investigators gathered an array of surveillance tapes from businesses near the attack site, intending to study through them frame by frame. Police also exhorted marathon spectators to share any video or photos they took in the hours before and just after the attacks — hoping for some clues about how and when the bombs were placed.
"There has to be hundreds, if not thousands, of photos and videos" that might aid investigators, said state police Col. Timothy Alben.
FBI agents searched an apartment in the Boston suburb of Revere, and left with paper bags, plastic trash bags and a duffel bag. According to one law enforcement official, the tenant had been tackled by a bystander, then police, as he ran from the scene of the explosions. But the man may simply have been running away to protect himself, said the official, who spoke on condition of anonymity because he wasn't authorized to release details of the investigation.
No conclusions could be drawn from the fact that there had been no claim of responsibility a day after the bombings, said counterterrorism intelligence specialist Ben Venzke, the founder and CEO of IntelCenter.
"It's not indicative of it being domestic or foreign — we can tell nothing from it," said Venzke.
He said the attempt to detonate an explosives-laden SUV in New York's Times Square in May 2010 was claimed within 24 hours by the Pakistani Taliban, while al-Qaida in the Arabian Peninsula took three days to claim responsibility for the thwarted 2009 attempt by the so-called underwear bomber to blow up a Delta airliner heading from Amsterdam to Detroit.
For now, Venzke said, it remains unclear whether the person or people who staged the bombings are trying to get away, laying low in the Boston area, or preparing a follow-up attack.
WHY? Until the perpetrators are identified, or a credible claim of responsibility emerges, it could be impossible to establish the motive for the attack.
Several foreign terrorist groups with long-standing hatred of America have threatened attacks on targets in the U.S., including al-Qaida, the Pakistani Taliban and al-Qaida in the Arabian Peninsula. According to Venzke, the latter group — in an article last year — suggested sports arenas and "annual social events" as targets for "mass slaughter of the population."
Some other analysts said timing of the blasts suggested a domestic attack.
Stratfor, a private global intelligence firm, said some domestic groups might have seen an appeal in staging an attack on April 15, when income taxes are due.
"There are also those who might see symbolism in proximity to the April 19, 1993, fire that ended the standoff with a religious cult near Waco, Texas, or the April 19, 1995, bombing of the federal building in Oklahoma City by Timothy McVeigh," Stratfor said.
Venzke said it would be irresponsible for investigators to ignore such anniversaries, even if they turn out to have no significance.
"For now they are useful but unconnected facts," he said. "You can't go further than that until you start to connect the threads."
HOW? Some details about the bombs' design emerged Tuesday, but not about where and when they were assembled, when they were positioned, or how they were set off.
Investigators had found pieces of black nylon from a bag or backpack and fragments of BBs and nails, possibly contained in a pressure cooker, said Richard DesLauriers, the FBI agent in charge in Boston. He said the items were sent to the FBI for analysis at Quantico, Va.
Pressure-cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 intelligence report by the FBI and Homeland Security. Also, one of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, the report said.
Venzke said such devices are relatively basic, compared to highly sophisticated bombs that have been used in some terrorist attacks.
However, Venzke said no investigative conclusions should be drawn from the use of such a basic device. He said a terrorist group such an al-Qaida, which has the capability to use sophisticated means of attack in some parts of the world, might opt for a much simpler device for an attack outside of its normal operating area.
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