BOSTON — 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.
Race officials said the boy had just hugged his father after he crossed the finish line, and was returning to where the sister and mother were standing a short distance away when a bomb exploded.
Six Boston hospitals treated the injured, several of whom were in critical condition. A doctor at Massachusetts General Hospital, where 30 of the victims were taken, said most of the serious wounds involved burns and lower body extremities.
Liz Norden of suburban Wakefield told the Boston Globe that two of her sons, both in their 30s, each had a leg amputated from the knee down.
President Obama, speaking on national television Monday night from the White House, assured the nation "we will find who did this and we will find out why they did this. Any responsible individuals, any responsible groups will feel the full weight of justice."
Investigators said they did not know if the bombings were connected in any way to the marathon being run on Patriots Day, a state holiday in Massachusetts to commemorate the start of the Revolutionary War with the battles of Lexington and Concord more than 235 years ago. It was also noted that Monday was the deadline for filing federal taxes.
The marathon is an iconic Boston event, attracting elite runners from across the world. Lelisa Desisa of Ethiopia won the men's division Monday, and Rita Jeptoo of Kenya the women's division. Both finished more than two hours before the bombing.
Details for this story were provided by the FBI, Massachusetts State Police and Boston Police Department.