Boston inquiry teams make progress on explosive devices

US law enforcement officers investigating the Boston Marathon bombs that left three dead and at least 176 injured have found traces of black nylon bags at both blast sites and possible home-made “pressure cooker” bombs at one.

Among the items recovered by investigators were pieces of black nylon that could be from a backpack and fragments of nails and “BBs”, a type of pellet, contained “possibly in a pressure cooker”, the FBI said. “The bag would have been heavy because of the components believed to be in it,” said Richard DesLauriers, the FBI chief in Boston.

The make-up of the bombs would not be known until after FBI headquarters in Quantico, Virginia, had forensically tested them. The range of suspects and motives “remains wide open,” the FBI said.

Investigators urged anyone who saw anyone researching bomb-making or heard explosions in remote areas to contact investigators. “Someone knows who did this,” said Mr DesLauriers.

The investigation update came shortly after Krystle Campbell (29) from the Boston suburb of Medford was named by local media as the second victim of the Monday afternoon bombings.

President Barack Obama described the attack as “a heinous and cowardly act” and “an act of terrorism” after the death of Martin Richard (8) from the Boston suburb of Dorchester was confirmed.

The boy was standing near the finish line with his family when they were caught in the first of two blasts about 15 seconds and 90m apart, close to the marathon’s finish line on one of Boston’s main streets.

The US remained on a heightened state of alert following the most high-profile attacks since the September 2001 attacks on New York City and Washington.

A terminal at LaGuardia Airport in New York was cleared over a suspicious package and a flight was stopped on the tarmac at Logan Airport in Boston to allow the authorities to examine a bag.

Pressure cooker explosives are similar to devices detonated in Afghanistan, India and Pakistan.