A major hunt continued Tuesday to determine who fashioned the two terrorist bombs that tore through crowds of innocent Boston Marathon spectators.
The bombs were created using common kitchen pressure-cookers packed with explosives and filled with nails. Investigators asked the public to provide photos and videos of the blast scene.
"It's very, very important," said FBI Special Agent Richard DesLauriers, at a late afternoon news conference. Hundreds of videos have already been handed over to police and more are sought as investigators intend to pore over the material in an attempt to determine who placed the bombs inside athletic bags or knapsacks and left them close to the finish line of the Boston marathon.
Without directly suggesting that the attack came from offshore, Mr. DesLauriers said the search for the terrorists "will be a worldwide."
"We will go to the ends of the Earth to identify the suspects responsible for this despicable crime," he said.
Although no group claimed responsibility and no motive was evident in the worst terrorist attack in the United States since Sept 11, 2001, security officials provided some details that sketched out the nature of the attack.
Only two bombs, both of which exploded, were planted by one or more pedestrians after the second explosives sweep of the area by law enforcement authorities – a time period of roughly three hours. Boston Police Commissioner Edward Davis confirmed there has been an explosives sweep of the entire race route early in the morning and then another roughly before the leading runners passed.
Early reports that other devices being discovered and disarmed were denied on Tuesday. "Two and only two explosive devices were found [Monday]," Massachusetts Governor Deval Patrick said at a joint news conference with city, state and federal police and security officials. "There were no other unexploded bombs found."
That will make investigators job slightly more difficult as they will need to re-construct one of the bombs from thousands of fragments to determine just how it was packed and triggered.
Police appealed to bystanders to give them photographs and videos taken of the bomb sites, especially in the minutes and hours before the explosions "There has to be hundreds, if not thousands, of photographs, videos and other observations that were made down at that finish line yesterday," said Timothy Alben, the Massachusetts State Police Superintendent.
The mid-afternoon blasts came roughly 10 seconds and 100 metres apart near the finish of the annual Boston race, which is staged on Patriots Day, the Massachusetts holiday that commemorates key battles in the American revolution.
Security experts said investigators will pore over the images – along with security footage from cameras mounted above Boston's busy Boylston Street – and painstakingly identify suspects and eliminate the innocent.
"They will have two objectives: to narrow the time frame of when the devices were planted and … they will one-by-one eliminate every person who came through that area, even if it is thousands or tens of thousands," said Christian Leuprecht, a terrorism expert who teaches at both Queen's University and Royal Military College of Canada in Kingston.
Police in Canada examined hundreds of hours of video and thousands of photos to eventually identify and arrest a handful of violent agitators after the June, 2010, G20 meeting and Prof. Leuprecht said the early call for photos in Boston suggested authorities wanted to amass as much photographic evidence as possible.
Meanwhile in Boston hospitals, doctors reported extracting tiny spheres – perhaps BB gun pellets or ball bearings – along with nails and other shrapnel from the injured, suggesting the bombers had packed the pressure cookers to inflict maximum damage. Earlier reports that other devices being discovered and disarmed were denied.
The use of pressure cookers as containers for bombs has worried U.S. and international security officials for years. A 2004 Homeland Security memo warns of the potential risks pressure cookers pose because they can easily be converted. "These bombs are made by placing TNT or other explosives in a pressure cooker and attaching a blasting cap at the top of the pressure cooker," the memo said. "Pressure cooker bombs are made with readily available materials and can be as simple or as complex as the builder decides. These types of devices can be initiated using simple electronic components including, but not limited to, digital watches, garage door openers, cellphones or pagers."
Pressure cooker bombs have been found in foiled plots in France and India but their use doesn't necessarily indicate that the Boston bombing was an international, rather than domestic, terrorism.
On Tuesday, President Barack Obama, who had avoided using the word terrorism in the hours after the blasts, made clear that's what it was. "Any time bombs are used to target innocent civilians, it is an act of terror," the President said. "What we don't yet know, however, is who carried out this attack or why, whether it was planned and executed by a terrorist organization – foreign or domestic – or was the act of a malevolent individual."
While the relatively simple and small bombs could suggest small groups or individuals, Prof. Leuprecht warned that even sophisticated terrorist organizations might use a "generic" bomb to cover their tracks. Still he suggested the relatively low casualty count and the fairly small and crude device "looks more like a measure of desperation than a really well-planned terrorist attack."
Even a crude bomb, like the pipe filled with explosives and nails planted by anti-abortion activist Eric Rudolph and that killed two and injured about 150 at the 1996 Summer Olympics in Atlanta, can cause the same magnitude of injury and death as the Boston blasts.