McALESTER — A massive police investigation was underway Tuesday for clues to who planted two powerful bombs near the finish line of the Boston Marathon, killing three spectators, including an 8-year-old boy, and injuring more than 170 people.
A federal official called it a "potential terrorism investigation." He said it appeared to be the work of more than one person.
FBI agents swarmed a high-rise apartment in the nearby suburb of Revere overnight, leaving with three large bags full of undisclosed material. Local officials described the apartment as the residence of a "person of interest" but no arrests were reported.
A 15-block area surrounding the scene of the bombings in the heart of downtown Boston was sealed with police tape, access restricted to residents who live there and hotel patrons. Bomb-sniffing dogs patroled streets, alleyways and subway stations.
Investigators were studying surveillance video from security cameras stationed in the area, television footage of the race and smartphone video submitted by spectators. Tuesday morning, officials made a public plea for photos and videos taken in the area of the explosions.
"We will go to the ends of the earth to identify the subject or subjects who are responsible for this despicable crime," said Richard DesLauriers, special agent in charge of the FBI's Boston office.
The twin explosions occurred more than four hours after the start of the 117th Boston Marathon, and after more than half of the 23,000 runners had completed the race. Police said the explosions happened 12 seconds apart at 2:50 p.m. Monday on Boyleston Street.
Officers sweeping the area screened other suspicious packages in the vicinity, but officials said they did not discover any other explosives.
Richard Martin, 8, of Boston's Dorchester neighborhood was identified as one of the three dead. His 6-year-old sister had a leg amputated and their mother suffered serious head injuries.