US authorities have released details of the explosives which ripped through the Boston Marathon, killing three people and injuring 170.

The FBI, which spearheads the investigation, believes that the bombs were "possibly contained in a pressure cooker device." Officials say the forensic evidence suggest that the bombs may have been carried in nylon bags or backpacks to launch the attack.

The twin blasts near the finish line of the marathon were the worst attack on US soil since the 9/11 attack in 2001. The explosives were placed with ball bearings and nails, which maimed several victims.

More than a dozen injured victims are still in a critical condition. Many of the victims have had their limbs amputated.

FBI Special Agent Richard DesLauriers told a news conference: "Among items partially recovered are pieces of black nylon which could be from a backpack and what appear to be fragments of BBs (ball bearings) and nails possibly contained in a pressure cooker device."

He went on: "This morning it was determined that both of the explosives were placed in a dark-coloured nylon bag or backpack. The bag would have been heavy because of the components believed to be in it."

DesLauriers added, however, that "the investigation is in its infancy. There are no claims of responsibility and the range of suspects and motives remains wide open."

The official pledged that US authorities would "go to the ends of the earth" to bring the perpetrators to justice.

No military outfit has claimed responsibility for the attack. Police have not held any suspect in custody as authorities said "the range of suspects and motives remain wide open."

US President Barack Obama will visit Boston on 18 April and will address an interfaith service. Obama earlier called the incident "an act of terror."

The president said the bombings were "heinous and cowardly" and insisted "it will take time... but we will find whoever harmed our citizens and we will bring them to justice."