Some investigators were seen leaving the Revere house early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.
Police said three people were killed. An 8-year-old boy 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.
Hospitals reported at least 144 people injured, at least 17 of them critically. At least eight children were being treated at hospitals.
Tim Davey of Richmond, Va., was with his wife, Lisa, and children near a medical tent that had been set up to care for fatigued runners when the injured began arriving. "They just started bringing people in with no limbs," he said.
"Most everybody was conscious," Lisa Davey said. "They were very dazed."
The Boston Marathon is one of the world's oldest and most prestigious races and about 23,000 runners participated. The race honored the victims of the Newtown, Conn., shooting with a special mile marker in Monday's race.
Boston Athletic Association president Joanne Flaminio previously said there was "special significance" to the fact that the race is 26.2 miles long and 26 people died at Sandy Hook Elementary School.
One of the city's biggest annual events, the race winds up near Copley Square, not far from the landmark Prudential Center and the Boston Public Library. It is held on Patriots Day, which commemorates the first battles of the American Revolution, at Concord and Lexington in 1775.
Boston Police Commissioner Edward Davis asked people to stay indoors or go back to their hotel rooms and avoid crowds as bomb squads methodically checked parcels and bags left along the race route. He said investigators didn't know whether the bombs were hidden in mailboxes or trash cans.