(Reuters) - A Boston-area apartment was one focus of a wide-ranging police investigation on Tuesday as authorities pursued clues into who carried out a bombing attack that killed three people and wounded more than 100 others at the storied Boston Marathon.
Police overnight searched an apartment in Revere, about 6 miles northeast of Boston, that was the residence of a person whose connection to the event is under investigation, law enforcement sources said.
A stretch of Boylston Street near the race's finish line 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.
It was the worst bombing on U.S. soil since security was tightened after the attacks of September 11, 2001.
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."
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.
Police searched the Revere residence of a Saudi Arabian student who was injured in the blasts, according to law enforcement sources. One of the sources said the student was the main lead investigators are looking into, but he has not been labeled a suspect.
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 counter-terrorism aide, Lisa Monaco.
The president is due to be briefed on the explosions later this morning by FBI Director Robert Mueller, Monaco and other senior staff.
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.
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.
MAJOR CITIES ON ALERT
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.
Runners who had traveled to the city for the race remained in shock on Tuesday morning.
Pat Monroe-DuPrey, of Winter Haven, Florida, ran with his wife, Laura, in a trip to mark their 10th anniversary after being married during the race.
He said he did not know what to make of the blast, which came as he was finishing the race in a state of exhaustion.
"You don't have a brain at 26 miles," Monroe-DuPrey said. "They got us off the course, and then I was panicking."
(Additional reporting by Mark Hosenball and Mark Felsenthal in Washington, Pritha Sarkar, Patrick Johnston, Martyn Herman and David Cutler in London and Gareth Jones in Berlin.; Editing by Christopher Wilson and Lisa Von Ahn)