Law enforcement agencies were scrambling on Tuesday to find out who carried out the Boston marathon blasts, which killed three people and injured more than 100 in an attack that caught US authorities by surprise.

FBI officials swooped on a residential building in Revere, eight miles north-east of Boston, late on Monday and removed a large bag from an apartment in the early hours of Tuesday morning. But in the immediate aftermath there are no arrests, no credible claims of responsibility and little information from police about who might have carried out the attack.