WBZ-TV reported late Monday that law enforcement officers were searching an apartment in the Boston suburb of Revere. Massachusetts State Police confirmed that a search warrant related to the investigation into the explosions was served Monday night in Revere but provided no further details.
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.
Hospitals reported at least 144 people injured, at least 17 of them critically. The victims' injuries included broken bones, shrapnel wounds and ruptured eardrums.
At Massachusetts General Hospital, Alisdair Conn, chief of emergency services, said: "This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population. This is what we expect from war."
Some 23,000 runners took part in the race, one of the world's oldest and most prestigious marathons.
One of Boston'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.
He said authorities had received "no specific intelligence that anything was going to happen" at the race.
The Federal Aviation Administration barred low-flying aircraft within 3.5 miles of the site.
"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."