BOSTON  — Two bombs exploded in the packed streets near the finish line of the Boston Marathon on Monday, killing three people and injuring more than 130 in a terrifying scene of shattered glass, bloodstained pavement and severed limbs at the world’s oldest and most prestigious marathon, authorities said.

A senior U.S. intelligence official said two other bombs were found near the end of the 26.2-mile course.

At the White House, President Barack Obama vowed that those responsible will “feel the full weight of justice.”

A White House official speaking on condition of anonymity because the investigation was still unfolding said the attack was being treated as an act of terrorism.

There was no word on the motive or who may have carried out the attack, and police said they had no suspects in custody. Authorities in Washington said there was no immediate claim of responsibility.

“They just started bringing people in with no limbs,” said runner Tim Davey, of Virginia. He said he and his wife, Lisa, tried to keep their children’s eyes shielded from the gruesome scene inside a medical tent that had been set up to care for fatigued runners, but “they saw a lot.”

“They just kept filling up with more and more casualties,” Lisa Davey said. “Most everybody was conscious. They were very dazed.”

Investigators ‘Zeroing In On Some People’ In Boston Bombings, Officials Say

Authorities investigating the Boston Marathon bombing said Wednesday they have developed “solid leads” and identified “a number of people” they want to talk to after viewing hundreds of hours of video, with one or two standing out.

Senior officials told NBC News that they were not prepared to characterize those individuals as suspects.

“We are zeroing in on some people,” an official said.

Investigators analyzing photos and video have found people carrying black backpacks and duffel bags and setting them down in the area where the bombs went off, officials said.

The investigators said they were focused on video taken in the area closest to the blasts, which killed three people and injured 176 near the finish line Monday. Senior officials in Boston also said the team of investigators on the ground is making “solid progress” and that forensics work on bomb parts continues.

Earlier in the day, doctors said they have pulled fragments as large as 2 inches, including pieces of wood, concrete and plastic, from the bodies of people wounded in the attack.

The injuries have been so severe that surgeons have operated a second time on some patients, even after amputations, to fight possible infection, said Dr. Peter Burke, the chief of trauma services at Boston Medical Center.

Investigators have said the two bombs were housed in metal containers — at least one a kitchen pressure cooker — and studded with metal, including fine nails or brads, to make the devices more lethal. Burke said that doctors are making the fragments available to police.