FBI 'following up on a variety of leads' to Boston bombings

As the FBI launched a hunt for those responsible for the Boston Marathon bombings, US President Barack Obama said: “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.”

There were no immediate claims of responsibility for the attack, the most serious in the US since the 9/11 atrocity.

Meanwhile, counter-terrorism expert Richard Barrett said the incident had hints of a right-wing attack rather than al-Qaida-inspired extremism.

Mr Barrett, the former United Nations co-ordinator for the al-Qaida and Taliban monitoring team, said it was too early to say who was to blame for the marathon blasts.

But Mr Barrett, who has served with MI5 and MI6, said the timing on Patriots’ Day and the relatively small size of the devices suggested the work of a domestic extremist.

“At the moment it looks more likely that it was a right-wing terrorist incident, rather than an al-Qaida attack because of the size of the devices.”

He added: “This happened on Patriots’ Day, it is also the day Americans are supposed to have their taxes in, and Boston is quite a symbolic city. These are all little indicators.”

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

Police were searching a flat in the Boston suburb of Revere as part of the investigation.

Massachusetts State Police confirmed that a search warrant was served last night but gave no more details.

Some investigators were seen leaving the property early today carrying brown paper bags, plastic binbags and a duffel bag.

Boston police commissioner Ed Davis said: "We're in the process of securing and processing the most complex crime scene that we've dealt with in the history of our department.''

Rick DesLauriers, special agent in charge of the FBI’s Boston Division, said: “Our mission is clear – to bring to justice those responsible for the marathon bombing.

“The American public wants answers, the citizens of the city of Boston and Massachusetts want and deserve answers.”

Mr DesLauriers confirmed US Immigration and Customs Enforcement (ICE) was a “key component” of the joint terrorism task force and were interviewing witnesses.

He added that he would not comment on who may or may not be held in custody.

He added: “This remains a very active investigation, are ongoing investigation in various locations throughout the area goes on.

“However, there are no known additional threats. We continue to interview various witnesses and process the crime scene, which could take some time.”

He said the FBI had received “voluminous” tip-offs over the last 18 hours.

He added: “The FBI is logically following up on a variety of leads.”

The Boston Marathon is one of the world’s oldest and most prestigious and about 23,000 runners took part, including hundreds from the UK and Ireland.

The race honoured the victims of the Newtown, Connecticut, school shooting with a special mile marker.

Boston Athletic Association president Joanne Flaminio previously said there was “special significance” to the fact that the race is 26.2 miles long and 26 people died at Sandy Hook Elementary School.

Scores of victims were hurt, many suffering shrapnel wounds, in the two blasts which rocked the finish line. Massachusetts General Hospital said “several amputations” had to be performed on survivors.

Horrifying images of injured spectators and blood-spattered pavements were caught as television cameras broadcast footage of the explosions.

Mayor Thomas Menino said: “This is a bad day for Boston. but that I think if we pull together we’ll get through it. We’re a strong city.”

Massachusetts senator Elizabeth Warren said: “The president of the United States has pledged his full support in all efforts to keep the city safe and to find the person who did this and bring them to justice.

“We did not have to reach out to the president, the president reached out to us. He called the governor, he called the mayor, he called the members of the delegation, because the president is actively involved here in responding.”

She added: “Boston will survive.”

Hospitals reported at least 144 people injured, at least 17 of them critically.

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.”

Tim Davey, of Richmond, Virginia, was with his wife, Lisa, and children near a medical tent that had been set up to care for fatigued runners when the injured began arriving.

“They just started bringing people in with no limbs,” he said.