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.

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.