Eight-year-old Martin Richard was among the dead, according to a person who talked to a friend of the family and spoke on condition of anonymity. The person said the boy's mother and sister were also injured as they waited for his father to finish the race.
A candle burned on the stoop of the family's single-family home in the city's Dorchester section Tuesday, and the word "Peace" was written in chalk on the front walkway.
Neighbor Betty Delorey said Martin loved to climb the neighborhood trees, and hop the fence outside his home.
Davis said there were 176 casualties at hospitals, at least 17 of whom were critically injured. At least eight children were being treated at hospitals.
The Boston Marathon is one of the world's oldest and most prestigious races and about 23,000 runners participated. Most of them had crossed the finish line by the time the bombs exploded, but thousands more were still completing the course.
The attack may have been timed for maximum bloodshed: The four-hour mark is typically a crowded time near the finish line because of the slow-but-steady recreational runners completing the race and because of all the friends and relatives clustered around to cheer them on.
Davis said authorities had received "no specific intelligence that anything was going to happen" at the race.
"We still don't know who did this or why," Obama said at the White House, adding, "Make no mistake: We will get to the bottom of this."
With scant official information to guide them, members of Congress said there was little or no doubt it was an act of terrorism.
"We just don't know whether it's foreign or domestic," said Rep. Michael McCaul, R-Texas, chairman of the House Committee on Homeland Security.