Three people were killed, including an 8-year-old boy, by two explosions just seconds apart near the finish line.
Police commissioner Ed Davis says 176 victims came to hospitals around Boston, and 17 of those are in critical condition.
Special Agent in Charge Richard DesLauriers says at a news conference there are no known additional threats.
Police commissioner Ed Davis says it is the most complex crime scene in history of the department.
Authorities are looking for amateur video and photographic evidence that can give clues to who set off the bombs.
Suffolk County District Attorney Daniel Conley says "what occurred in Boston was an act of cowardice."
BOSTON (AP) - The young victim of the Boston Marathon bombings is being remembered as a vivacious boy who loved to run and climb.
Eight-year-old Martin Richard was among the three people killed in the explosions. That's according to a person who spoke with a friend of the family.
A candle burned on the stoop of the family's single-family home in the city's Dorchester section today, and the word "Peace" was written in chalk on the front walkway.
Iranian state TV says at least 40 people have been killed by a major earthquake near the Iran-Pakistan border.
Press TV gave no further details on the extent of damage in the sparsely populated areas. But the quake shook buildings as far away as New Delhi and Gulf cities of Dubai and Bahrain.
Iran's seismological center said the 7.5 magnitude earthquake was centered near Saravan, a sparsely populated area about 48 kilometers (26 miles) from the Pakistani border. The U.S. Geological Survey put the preliminary magnitude at 7.8 and at a depth of 15.2 kilometers (nine miles).
The quake struck less than a week after a 6.1 magnitude quake hit near Bushehr, on Iran's Persian Gulf coast, killing at least 37 people.