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.

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.

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.

Pressure cookers

The explosions at the marathon took place about 10 seconds and about 90 meters apart, knocking spectators and at least one runner off their feet.

The explosives were made of pressure cookers packed with gunpowder and shrapnel, law enforcement sources said.

The sources said the explosives were in six-litre pressure cookers and placed in black duffel bags that were placed on the ground.

The bags contained shards of metal, nails and ball bearings.

Al Jazeera's Alan Fisher said such a bomb was set to explode by using a mobile phone and had been used in Iraq and Afghanistan.

"The police were saying that there were traces of pressure cooker found at the site," he said.

"The suggestion is that the pressure cooker was put in a backpack and placed on the ground, and that is why we've got so many lower body injuries."

Our correspondent said that would explain why there was a small blast which caused a great deal of damage to the people nearby, as the shrapnel spread over a wide area.

Deval Patrick, governor of Massachusetts, said that no unexploded bombs were found at the marathon. He said the only explosives were the ones that went off on Monday.

SOURCE: http://www.aljazeera.com/news/americas/2013/04/2013416152112944580.html