Investigators continue to scour for leads following yesterday's Boston Marathon bomb attack that killed three people, including eight-year-old Martin Richard.

Martin Richard was killed while waiting with his family for his father to complete the Boston Marathon, his mother and sister are in a serious condition in hospital.

At least 176 people were injured in the attack, which is being as treated an act of terror by police.

Officers are now appealing for private video, audio and still images of the attack to be handed over.

US Defense Secretary Chuck Hagel called the events a "cruel act of terror" and promised that a thorough investigation will determine whether the perpetrators were foreign or domestic. The Pentagon chief said government officials currently do not know who is responsible or why the race was targeted.

The Queen has sent a private message to US President Barack Obama, Buckingham Palace has confirmed. It is believed she has offered her condolences.

The eight-year-old boy who died in the Boston Marathon bomb attack was named today as Martin Richard.

He was waiting with his mother, brother and sisters for his father William to complete the marathon. The family are from the Boston suburb Dorchester. Mother Denise Richard and one sister are in hospital with serious injuries.

At least 17 were critically injured when two bomb blasts rocked the finish line, and many were taken to hospital suffering shrapnel wounds. A number of amputations took place at Massachusetts general hospital. Three people, including Martin Richard, were killed.

Police searched a flat in the Boston suburb of Revere as part of their investigation with a search warrant that was served last night.

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."

He confirmed there are no known additional threats at present and police are leading a "very active investigation" in various locations throughout the area. He said the FBI had received "voluminous" tip-offs over the last 18 hours.

Boston police commissioner Ed Davis has confirmed there were two "simultaneous explosions" near the finish line of the 26-mile event.

The explosions came roughly four hours into the race, a time when a large numbers of runners would be crossing the line. Police later said that early reports of unexploded bombs were found near the end of the course were incorrect.

President Barack Obama, speaking from the White House, avoided using the words "terror" or "terrorism", saying officials still do not know "who did this or why". The president went on to say those responsible will feel the "full weight of justice".

A White House official has said the explosions were being treated as terrorism.

Graphic images of injured spectators and pavements covered in blood were broadcast on American television.

Participants were seen lying on the ground as the blast tore through the finish line, sending smoke and debris through the air.

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A list of competitors on the Boston Marathon website showed that hundreds of British and Irish runners were expected to take part in the race.

David Cameron took to Twitter to offer his support: "The scenes from Boston are shocking and horrific - my thoughts are with all those who have been affected."

The London Marathon on Sunday will face stricter security following the explosions, Met Police Chief Superintendent Julia Pendry has confirmed.

Sports Minister Hugh Robertson insisted he was "absolutely confident" that the London event could be kept safe.

Speaking on BBC Radio 4's Today programme, he said: "These are balance of judgments but we are absolutely confident here that we can keep the event safe and secure.

"I think this is one of those incidents where the best way to show solidarity with Boston is to continue and send a very clear message to those responsible."

The chief executive of the London Marathon, Nick Bitel, said he was in touch with the police as soon as he heard the news.

"Our immediate thoughts are with the people there and their families. It is a very sad day for athletics and for our friends and colleagues in marathon running."

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