US authorities have released the first pictures of the explosive devices used in the twin blasts which hit the Boston Marathon on Monday.
The images show twisted pieces of a metal container, wires, a battery and a small circuit board which exploded close to the finish line at the race.
In another, a piece of charred wire can be seen attached to a small box and a twisted metal lid with bolts.
The FBI said on Tuesday that a pressure cooker may have been used to build the bombs, with nails, ball bearings and other metal packed around the explosive.
The device may also have been hidden inside rucksacks placed on the ground near the finish line.
Richard DesLauriers, FBI agent in charge said the investigation to find those responsible would be worldwide.
He vowed: "We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime."
More than 1,000 officers are working on the investigation which agents have said in the largest the Boston bureau has ever worked on.
Pressure cooker bombs have been used in attacks in Afghanistan, India, Nepal and Pakistan, according to a July 2010 FBI report.
But the techniques of making them is understood to be known to domestic US extremists.
Officials said that there was no indication that al Qaeda or other foreign extremist organisations were behind the attack, but they added the investigation was still at an early stage.
It is not yet known what was used to set off the devices.
Special agent DesLauriers said experts would reconstruct the devices at the FBI laboratory in Quantico, Virginia.
The FBI also appealed for anyone who was in the area of the marathon or Boston airport in the last few days to send in any pictures they may have taken.
Investigators are already understood to be examining 6,000 movies-worth of CCTV footage from cameras in the area.
Websites and newspapers were already featuring images which they claimed showed people suspected of carrying out the attacks or the devices.
The FBI said it was looking at one sent to a local TV station which appeared to show a bag next to a mailbox in the area where one of the bombs went off.
Jason Pack, FBI spokesman in Boston, said: "We're taking a look at hundreds of photos and that's one of them."
Three people were killed and more than 170 others injured after the two explosions around four hours into the famous marathon.
Two of those killed were Krystle Campbell, 29, and eight-year-old Martin Richard.
The third is understood to be a Chinese graduate student who has been named locally and in China, but not named officially.
Nine children were among the injured, which were aged between two and 71.
Doctors have revealed the extent of the injuries suffered by those caught in the blasts, including details of a nine-year-old girl who lost her leg and a 10-year-old boy who suffered deep shrapnel wounds.
George Velmahos, of Massachusetts General Hospital, said: "These bombs contained small metallic fragments more consistent with pellets and other small pieces of metal, but also spiked points that resembled nails without heads."
A total of 13 people have had to have limbs amputated and others are at risk of losing legs following the blasts. Seventeen remain in a critical condition.