The full might of America's police and security services was today focused on finding out who planted the Boston marathon bombs, and why.

No one has claimed responsibility for the devastating attack and president Barack Obama was careful not to use the words "terror" or "terrorism" as he spoke at the White House after the bombings that killed three people and injured scores, but one official said it was being treated as an act of terrorism.

"We will find out who did this. We'll find out why they did this," Mr Obama said. "Any responsible individuals, any responsible groups, will feel the full weight of justice."

"We still don't know who did this or why," Mr Obama said, adding, "Make no mistake: We will get to the bottom of this."

With scant official information to guide them, members of Congress said there was little or no doubt it was an act of terrorism.

"We just don't know whether it's foreign or domestic," said Congressman Michael McCaul, chairman of the House Committee on Homeland Security.

The Pakistani Taliban, which has threatened attacks in the United States because of its support for the Pakistani government, denied any part in the bombings.

The FBI took charge of the investigation, searching a home in suburban Boston and appealing for any video, audio and still images taken by marathon spectators.

Two suspected unexploded bombs were found near the end of the course as part of what appeared to be a coordinated attack, but they were safely disarmed, according to a senior U.S. intelligence official.

Police searched an apartment in the Boston suburb of Revere. Some were seen leaving the building carrying brown paper bags, plastic trash bags and a duffel bag.

Boston Police Commissioner Edward Davis said authorities had received "no specific intelligence that anything was going to happen" at the race.

A few miles away from the finish line and around the same time, a fire broke out at the John F. Kennedy Library. The police commissioner said that it may have been caused by an incendiary device but that it was not clear whether it was related to the bombings.

The bombings occurred about four hours into the race and two hours after the men's winner crossed the finish line. By that point, more than 17,000 of the athletes had finished the marathon, but thousands more were still running.

The attack may have been timed for maximum carnage: The four-hour mark is typically a crowded time near the finish line because of the slow-but-steady recreational runners completing the race and because of all the friends and relatives clustered around to cheer them on.

 

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