Reports that a suspect has been detained over Monday's Boston marathon bombings have been denied by US officials.

The Associated Press and CNN cited law enforcement officials as saying someone was in custody, but the Boston police department denied the reports.

Investigators have been sifting through thousands of pieces of evidence, ranging from video recorded on mobile phones to fragments of shrapnel removed from the victims' legs. More than 170 people were wounded in the attack.

Earlier, the Associated Press quoted a source close to the investigation as saying that the bombs consisted of explosives placed in 1.6-gallon (6-litre) pressure cookers, one with shards of metal and ball bearings, the other with nails.

Nobody has claimed responsibility for the bombs, which a source said were placed in black bags and left on the ground.

It has been reported that a circuit board and battery pack - parts of a triggering mechanism - were recovered.