BOSTON (Reuters) - A large area of downtown Boston remained cordoned off by police on Tuesday as authorities pursued an investigation into the two bombs that exploded at the finish of the Boston Marathon a day earlier, killing three people and wounding more than 100.
A stretch of Boylston Street and the blocks around it were closed to traffic as police searched for evidence of the identity of who placed the bombs packed with ball bearings to maximize casualties.
The White House said the bombings would be treated as "an act of terror" and President Barack Obama vowed that those responsible would "feel the full weight of justice."
The attack was the worst on American soil since September 11, 2001, when al Qaeda militants flew hijacked airliners into New York's World Trade Center and the Pentagon, killing nearly 3,000 people.
In Boston, dozens of police and national guard vehicles were parked around the cordoned-off area, which was empty of cars and pedestrians as authorities hunted for clues.
A banner that had marked the race's finish line still hung over the deserted street.
Local media reported that police searched an apartment in Revere, a city about six miles (10 km) northeast of Boston. Katherine Gulotta, a spokeswoman for the FBI, which has taken over the lead in the investigation, declined to confirm or deny the reports. She said police planned to brief the media at 9:30 a.m. (1330 GMT).
Obama was updated on the investigation overnight by his homeland security and counterterrorism aide, Lisa Monaco, a White House official said.
"The president made clear that he expects to be kept up to date on any developments," the official said.
Obama is due to be briefed on the explosions later this morning by FBI Director Robert Mueller, Monaco, and other senior staff, the official said.
White House officials and investigators said it was too early to say whether the Boston attacks were carried out by a foreign or homegrown group or to identify a motive.
In 1995, far-right militant and U.S. citizen Timothy McVeigh set off a massive truck bomb that destroyed the Federal Building in Oklahoma City, killing 168 people.
Two years earlier, Islamist militants bombed the twin towers of the World Trade Center, killing six people and wounding more than 1,000.
Hospitals in the Boston area were planning surgeries for some of the victims, many of whom sustained lower leg injuries in the blasts, said Peter Fagenholz, a trauma surgeon at Massachusetts General Hospital.
"We're seeing a lot of shrapnel injuries" from small metal debris, Fagenholz told reporters outside the hospital. Doctors treated 29 people, of whom eight were in a critical condition.
An 8-year-old boy was among the dead, the Boston Globe reported, citing two law enforcement sources briefed on the investigation. A 2-year-old was being treated at Boston Children's Hospital for a head wound, the hospital said.
The blasts put police on alert in major cities across the United States, including Washington and New York City, the sites of the September 11 attacks.
The annual Boston Marathon, held since 1897, attracts an estimated half-million spectators and some 20,000 participants every year.
In Britain, organizers said the London Marathon would go ahead on Sunday despite the Boston attack, but security was being reviewed.
The Madrid Marathon also planned to proceed on Sunday, but security plans were under review, a Spanish official said.
"After what happened in Boston we'll have to look into whether we need to review our plans. Since yesterday we are coordinating with municipal security and local government," Pedro Rumbao, director of the Madrid marathon, told Spanish National Radio.
German Chancellor Angela Merkel Joined world leaders in condemning the blasts.
"Nothing can justify such an insidious attack on people who had come together for a peaceful sports event. I hope that the person or people guilty (of this attack) can be brought to justice," she said in a statement. (Aditional reporting by Mark Felsenthal in Washington, Pritha Sarkar, Patrick Johnston, Martyn Herman and David Cutler in London and Gareth Jones in Berlin.; Writing by Andrew Heavens; editing by Jon Boyle and Christopher Wilson)