Boston bombs made from pressure cookers, shrapnel

Investigators combing through the grim aftermath of the deadly Boston Marathon terrorist attack have found evidence that timing devices were used on Monday to detonate the bombs that ripped through race spectators, an official briefed on the investigation into the first large-scale bombing in the US since the 9/11 attacks, said.

Working with fragments painstakingly gathered at what is considered the city's largest-ever crime scene, they also determined that the two bombs were probably fashioned from 6-litre pressure cookers, filled with nails and small ball bearings, like buckshot, to increase the carnage, and then hidden in black nylon bags or backpacks and left on the ground, The Boston Globe reported.

FBI bomb experts at the FBI Academy in Quantico, will try to rebuild the devices from fragments that include a circuit board that indicated the bombs were detonated on a timer, rather than remote control.

The devices used are typical of the "lone wolf", the solo terrorist, who builds a bomb on his own by following a widely available formula, CNN reported.

It is also a recipe that has been adopted by extreme right-wing individuals in the US.

However, more than 48 hours after the horrific bomb blasts in Boston, which killed three people and injured more than 170, investigators had no clue as to who were - an individual or any group - were behind this act of terror.

President Barack Obama will travel to Boston tomorrow for a memorial.

While every possible lead is being explored and probed, investigators so far have not been able to trace the link of this terrorist act to a foreign country, sources said.

"The pressure cooker device is commonly used in Afghanistan and Pakistan and India. They have used those for IEDs (improvised explosive devices). But that's because it is a cooking utensil that is readily available in those countries. Not so much here," Republican Senator James Risch told the Fox news in an interview.

"But, having said that, you can offset that against the fact that if you go on the Internet to learn how to build a bomb, pressure cookers are one of the ways that is commonly put out on the Internet as a way to make a bomb," Risch said.

"And pressure cookers can be obtained here relatively easily and without usually raising much alarm in people's eyes," he said.

The Senator, who was in a number of classified briefings, observed that the attack is much more reminiscent of the event in 1996 at the Atlanta Olympics, where a man by the name of Eric Rudolph exploded a bomb that was inside a backpack and did the damage that he did.

"As you recall, it took seven years to catch him," he said.

Risch said had it been a foreign terrorist outfit, they would have been very quick to step forward and take responsibility for it for a lot of different reasons.

Number one, it gets their message out. Number two, they use it for recruiting and that sort of thing, he said.

"That did not happen here. That's not always the case, but most likely the case. If you look at the reverse side of that, if it is a lone wolf-type of an attack, or if it's an attack from some deranged American person, they generally do not come forward and take credit for it. And, of course, that's what happened. No one has come forward," the Senator said.

The FBI at a news conference confirmed that the explosive devices possibly used pressure cooker.

"Possibly a pressure cooker. Black nylon bags, possibly at both sites. We're postulating that they needed to be heavy bags to carry the explosive devices inside of them. They would not be light bags," FBI Boston chief Rick DesLauriers told reporters.

According to the FBI there are multiple pieces of information that they are trying to construct together.

"As far as precisely what evidence was found at what location, I wouldn't want to go into the details of that right now, but there are multiple pieces of evidence that are at the crime scene, as you might well imagine from the serious, tragic nature of this event. That analysis is ongoing right now," DesLauriers said.

"Information that could possibly be a pressure cooker was found at the site, and we are putting that out to the public in an attempt to generate any lead information," he said in response to a question.

Among items partially recovered are pieces of black nylon, which could be from a backpack, and what appear to be fragments of ball bearings and nails, possibly contained a pressure-cooker device, DesLauriers said.

"We are expediting this last evidence to our laboratory in Quantico, Virginia, for a complete and thorough analysis," he said.

The investigation has also determined that both of the explosives were placed in a dark-colored nylon bag or backpack.

"The bag would have been heavy because of the components believed to be in it. At this point it is difficult to determine specific components used until we can eliminate other factors which may have already been present in the environment. In fact, we won't know with some certainty until the laboratory completes its final review," DesLauriers said.

More than 1,000 law enforcement officials have been pressed into the Boston Marathon bomb blast.

"They began canvassing sources, reviewing government and public source databases and conducting interviews with eyewitnesses and others to determine who was responsible for this crime. We are doing this methodically, carefully yet with a sense of urgency," he said.

The FBI, so far, he said, has received more than 2,000 tips, many of which have been reviewed, analysed and vetted.

"Regarding who might be suspected of this event, the investigation is in its infancy," he said. "At this time there are no claims of responsibility. The range of suspects and motives remains wide open," he added.

"Importantly, the person who did this is someone's friend, neighbour, co-worker or relative. We are asking anyone who may have heard someone speak about the marathon or the date of April 15th in any way that indicated that he or she may target the event to call us. Someone knows who did this," the FBI official said.

"We don't have any definitive information regarding the suspects or subjects. It could be a person; it could be persons. We have not reached any conclusions at all in that regard," DesLauriers said.