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.
"This was a heinous and cowardly act," President Barack Obama said in a morning statement to the press, "and given what we now know about what took place, the FBI is investigating it as an act of terrorism."
Officials did not know who planted the bombs, how many were involved or what the motive was, he said.
Hours earlier, FBI agents swarmed a high-rise apartment in the nearby suburb of Revere, 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.