Federal investigators had a major breakthrough in the investigation of the Boston Marathon terror attack.
They say they've recovered the partial remains of one of the bombs: a medium-sized pressure cooker packed with wires, a circuit board, nails and ball bearings.
"It appears this device may have been dropped in a backpack, either inside the trash cans or right outside the trash cans," Rep. Mike McCaul, R-Texas, said.
Counter-terrorism experts immediately recognized the design as a type used by al Qaeda operatives around the world.
Instructions on making a pressure cooker bomb were provided in an al Qaeda magazine article entitled, "How to Make a Bomb in the Kitchen of Your Mom."
But while the design is clearly al Qaeda's, no individual or group has taken responsibility for the bombing that killed three, including an 8-year-old boy, and injured more than 140, many suffering loss of limbs.
FBI investigators are urging the public to come forward with pictures or any other information that could lead to an arrest.
"The person who did this is someone's friend, neighbor, co-worker or relative. Someone knows who did this," said Rick DesLauriers, the Boston FBI special agent in charge of the investigation.
As authorities work to find the terrorist before he escapes, Boston remains a city in pain. But despite this time of mourning, residents say they're still "Boston strong."
Organizers have already announced that next year's Boston Marathon will go on, and locals say they will too.
"As an American, my heart is broken, but as a Bostonian my resolve for the pursuit of freedom and liberty has never been stronger," one resident said.
"We're a strong people," another resident, Dennis Pinto, said. "We're not going to let anybody push us around. You mess with one of us, you mess with all of us."
President Obama referred to the bombing as an act of terror on Tuesday. He scheduled to travel to Boston Thursday for a service honoring the victims.