Obama: FBI treating 'cowardly' bombings as terrorist attack

US President Barack Obama has said they do not know who carried out the Boston marathon bombings or why.

Three people, including an eight-year-old boy, were killed when two devices exploded close to the finish line of the race.

More than 170 others were injured, a number of them critically.

President Obama said the FBI is treating it as a terrorist attack but it is not clear who was behind it.

“This was a heinous and cowardly act,” he said. “The FBI is now investigating this as an act of terrorism.”

But he added: “The American people refused to be terrorised. What the world saw yesterday in the aftermath of the explosions were stories of heroism and kindness and generosity and love.”

"They did some severe damage, we do not know who did them.

"We do not know whether this is an act of an organisation or an individual or individuals.

"We don’t have a sense of motive yet, so everything else at this point is speculation."

The investigation into the Boston marathon bombings is in full swing with a police raid on a private home and an appeal for any video, audio or still images of the blasts.

The president promised: “We will find out who did this. We’ll find out why they did this. Any responsible individuals, any responsible groups, will feel the full weight of justice.”

“We still don’t know who did this or why,” he 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 Republican Michael McCaul, chairman of the House Committee on Homeland Security.

Richard DesLauriers, FBI agent in charge in Boston, said that investigators had received “voluminous tips” and were interviewing witnesses and were analysing the crime scene.

“We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice,” he said.

One European security official said 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 did not want to be named.

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

Defence Secretary Chuck Hagel called the bombing a “cruel act of terror.” He said that any event with explosive devices is clearly an act of terror and he promised that a thorough investigation will determine whether the perpetrators were foreign or domestic. The Pentagon chief vowed that those responsible will be brought to justice.

Massachusetts governor Deval Patrick said no unexploded bombs were found at the marathon site. He said the only explosives were the ones that went off.

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

Police refused to give any specifics on the bombs and say, for example, where they might have been hidden or whether they were packed with shrapnel for maximum carnage, as is often the case in terror bombings overseas.

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.