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Expert: Long Investigation Likely Needed To Get To Bottom Of Boston Explosions

NEW YORK (CBSNewYork) -- The Boston Marathon bombing has been deemed an act of terrorism, but whether it was an international or domestic act remained undefined Monday night.

As CBS 2 investigative reporter Tamara Leitner reported, a painstaking probe to determine the motive for the attacks was in its early stages.

At least three people were killed, and 144 were injured, after the two explosions went off near the finish line of the marathon, police said. The fact that the bombs detonated two blocks apart means investigators will have to painstakingly search and preserve two crime scenes.

"This is going to be a long crime scene to process," said CBS News senior correspondent John Miller, a former assistant FBI director. "When a device functions as it's supposed to, it breaks itself into a million pieces. They're going to want to get every single one of those, because it can literally be down to how the wire is twisted, or what kind of timing device is used that will give them the bomber's signature."

The Boston Police Bomb Squad was on the front line, but also investigating was the FBI, the Bureau of Alcohol, Tobacco, Firearms and Explosives, the U.S. Armed Forces, and essentially the entire intelligence community.

"I watched those explosions a number of times very carefully, and those are not terribly large bombs, and those fragments are going to be collectible," Miller said. "What they do is fan out in a crime scene like that and find the furthest possible fragment of the bomb and then go out three times further than that, figuring there are fragments further they can't see, and they'll do a grid search where they'll pick up something as small as a piece of copper wire."

CBS 2 has learned police recovered a third bomb, which did not detonate. A counterterrorism expert said that could be key in providing information on where the device was made, what was used and possibly who was behind constructing the explosive devices.

The expert also said the back-to-back explosions are a technique that terrorists use. Once the first explosion goes off, people rush to help. At that point, a second explosion may go off, ensuring more casualties.

Finally, police Monday night were already reviewing all surveillance cameras in the area. CBS 2 was told a man was seen on a camera -- only from the back -- but carrying two backpacks.

Investigators want to know who he is, and what he did with the backpacks. They said they will use other cameras to trace his path.

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