'Who' and 'why' remain unknown
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 worst incident of terrorism in the United States since the 9/11 attacks of 2001:
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.
Counterterrorism intelligence specialist Ben Venzke, the founder and CEO of IntelCenter, said no conclusions could be drawn from the fact that there had been no claim of responsibility a day after the bombings.
"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 Paris 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 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 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 or when they were positioned.
According to a person briefed on the investigation, the bombs were fashioned out of pressure cookers and packed with metal shards, nails and ball bearings to inflict maximum carnage.
The person, who spoke on condition of anonymity because the investigation was ongoing, said devices were hidden in black duffel bags and left on the ground.
Similar 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.