Three fatal victims

The bombs in Boston killed an 8-year old boy, Martin Richard; a 29-year-old woman, Krystle Campbell, and a Boston University graduate student and Chinese citizen, Lu Lingzi.

The crowded scene along the race course in central Boston on Monday was recorded by surveillance cameras and media outlets, providing investigators with significant video of the area before and after the two blasts.

Investigators search for evidence on the rooftop of a building located above the site of a bomb blast on Boylston Street two-days after multiple explosions at the Boston Marathon killed three and injured 176 in Boston, Massachusetts April 17, 2013. [Photo/Agencies]

Investigators were also searching through thousands of pieces of evidence from cellphone pictures to shrapnel pulled from victims' legs.    

Based on the shards of metal, fabric, wires and a battery recovered at the scene, the focus turned to whoever may have placed homemade bombs in pressure cooker pots and taken them in heavy black nylon bags to the finish line of the world-famous race watched by thousands of spectators.

Streets around the bombing site remained closed to traffic and pedestrians on Wednesday, with police continuing their work.

No one had claimed responsibility for the attack.

"Whether it's homegrown or foreign, we just don't know yet. And so I'm not going to contribute to any speculation on that," said US Secretary of State John Kerry, who until January was Massachusetts' senior senator. "It's just hard to believe that a Patriots' Day holiday, which is normally such time of festivities, turned into bloody mayhem."

Security officials who spoke on condition of anonymity said instructions for building pressure-cooker bombs similar to the ones used in Boston can be found on the internet and are relatively primitive.

Pressure cookers had also been discovered in numerous foiled attack plots in both the United States and overseas in recent years, including the failed bombing attempt in New York's Times Square on May 1, 2010, the officials said.