American investigators have found a piece of circuit board and the lid of one of the pressure cookers believed to have been used as the explosive device from a rooftop near the Boston Marathon blasts site which is expected to provide vital clues to crack the case.
One of the two bombs that exploded near the finish line of the Boston Marathon is believed to have been hidden in a pressure cooker inside a backpack, the FBI said in a joint intelligence bulletin on Wednesday.
The device also had fragments that may have included nails and ball bearings, the agency said.
A CNN report said such clues are moving the investigation forward but reveal less about whether the attack was an act of domestic or foreign terrorism.
The second bomb was also hidden in a metal container, but it was not clear whether it too was in a pressure cooker, the FBI said.
The twin blasts, the first large-scale bombing since the 9/11 terror attacks on the US soil in 2001, ripped through the cheering crowd at the finish line of the Marathon on Monday, leaving three persons dead and 176 others wounded.
Special agent in charge of FBI's Boston division Richard DesLauriers said several "physical items" related to the bombings have been recovered from the blast site and those have been sent to the FBI's Laboratory in Virginia where specialized examiners would reconstruct the device and determine its makeup and components.
Among items partially recovered are pieces of black nylon which could be from a backpack - fragments of nails possibly contained in a pressure cooker device. The pieces suggest each of the devices was 6 liters in volume.
The recovered parts also include part of a circuit board, which might have been used to detonate a device.
Doctors treating the wounded at several local hospitals said they have found metal pellets and nails inside patients' bodies.