U.S. President Barack Obama on Tuesday called the bombings that killed three andwounded more than 170 people at the Boston Marathon a day earlier an "act of terror," but said it was not clear yet whether the twin blasts were the work of a foreign or domestic group or a "malevolent individual."
In an appearance in the White House briefing room, Obama urged Americans to be vigilant and to watch for suspicious activity in the wake of the attack.
U.S. Secretary of Defense Secretary Chuck Hagel on Tuesday also called the bombings a "cruel act of terror," saying an any event with explosive devices could clearly be defined as such.
He vowed that those responsible will be brought to justice, but added that government officials still do not know who is responsible or why the historic race was targeted Monday.
A person briefed on the Boston Marathon investigations said Tuesday that the explosives were in 6-liter pressure cookers and placed in black duffelbags.That source the explosives were placed on the ground and contained shards of metal, nails and ball bearings. The person spoke on the condition of anonymity because the investigation was continuing.
The source added that law enforcement officials have some of the bomb components but did not yet know what was used to set off the explosives.
A Boston-area apartment was one focus of a wide-ranging police investigation on Tuesday as authorities pursued clues into who carried out the bombing at the storied Boston Marathon.The Boston police commissioner said no suspects were being held yet in connection with the bombing.
Police overnight searched an apartment in Revere, about 6 miles (10 km) 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.No additional explosive devices were found on the scene, Massachusetts Governor Deval Patrick said on Tuesday.
It was the worst bombing on U.S. soil since security was tightened after the attacks of Sept. 11, 2001.
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 hungover the deserted street.
Police searched the Revere residence of a Saudi Arabians tudent 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.
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 Tuesday 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.
A total of 176 people were treated at area hospitals as a result of the attacks and 17 are in critical condition, Boston Police Commissioner Ed Davis told a news conference.
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 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.
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."
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