Images show a backpack and apparent fragments of a pressure cooker as bureau probes attack that left three people dead.
The explosions at the marathon took place about 10 seconds and about 90 metres apart, knocking spectators and at least one runner off their feet.
Photo: Image of a bag that is believed to have held one of the Boston Marathon bombs (FBI)
Law enforcement officials in the US say they have recovered forensic evidence that suggests the two explosive devices which ripped through participants of the Boston Marathon on Monday may have been inside heavy black nylon bags.
Investigators who found pieces of black nylon at the scene suspect the bombs used in the attack were in dark-coloured bags that would have been heavy to carry.
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Speaking at a joint law enforcement news conference on Tuesday, Richard DesLauriers, the FBI special agent in charge of the case, said that investigators had received "voluminous tips" and were interviewing witnesses and were analysing the crime scene.
DesLauriers pledged "we will go to the ends of the Earth" to find whoever carried out the deadly attack on one of the city's most famous civic holidays, Patriots Day.
Authorities served a warrant on a suburban Boston home and appealed for any images or audio of the blasts.
Amid heightened security across the country, Harry Reid, the Senate majority leader, said a letter containing ricin or another poision had been sent to the office of Senator Roger Wicker of Mississippi.
Earlier, Barack Obama, the US president, confirmed the FBI is investigating the Boston bombings as an "act of terror", Obama said that investigators had yet to find a culprit for the two bombings that killed three people and injured 176.
The president called the bombing "a heinous and cowardly act" used to target innocent civilians.
CFN - California Fire News 2013