A large area of downtown Boston was cordoned off by police on Tuesday as authorities hunted for a suspect in the deadly bombings at the finish of the Boston Marathon that killed three people and wounded 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 Sept. 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 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 a morning briefing.
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 Tuesday 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.
The Boston Marathon is run on Patriots Day, a holiday in Massachusetts that commemorates the opening battles in Lexington and Concord of the Revolutionary War. Patriots Day had been designated on April 19 and is now observed on the third Monday of April.
On April 19, 1995, far-right militant 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.
A European security official said Tuesday initial evidence indicates that the attacks were not the work of suicide bombers.
"So far, investigators believe it was not the work of suicide bombers, but it is still too early to rule it out completely," said the official, who spoke from the United States on condition of anonymity because he was not authorized to speak about the U.S. investigation.
The Pakistani Taliban, which has threatened attacks in the United States because of its support for the Pakistani government, on Tuesday denied any role in the marathon bombings.
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, the sites of the Sept. 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.
The fiery explosions took place about 10 seconds and about 100 yards apart, knocking spectators and at least one runner off their feet, shattering windows and sending dense plumes of smoke rising over the street and through the fluttering national flags lining the route.
Blood stained the pavement, and huge shards were missing from window panes as high as three stories. Victims suffered broken bones, shrapnel wounds and ruptured eardrums.
Roupen Bastajian, a state trooper from Smithfield, R.I., had just finished the race when he heard the explosions.
"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs. A lot of people amputated. ... At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."
Hospitals reported at least 144 people injured, at least 17 of them critically. At least eight children were being treated at hospitals.
At Massachusetts General Hospital, Alasdair 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."