NBC News   |  April 15, 2013

How many explosive devices, and where?

NBC News' Brian Williams speaks to Pete Williams about the latest in the investigations of the Boston Marathon bombings, including the potential number and locations of explosive devices.

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This content comes from Closed Captioning that was broadcast along with this program.

>> day long pete williams has been working on this story from our washington bureau and pete has more on it for us tonight. pete , good evening.

>> brian, good evening. tonight investigators know that the attack involved two bombs. how many more is unclear. at least five other packages found on the street were deemed suspicious and were destroyed. there were many packages to check officials say because after the explosions many people simply dropped what they were carrying and ran leaving their packages behind. we have heard conflicting accounts tonight about whether there were also some unexploded bombs , some officials say tonight, yes, that two other packages were found that contained some form of explosive explosives but other officials say, no, just two bombs. the ones that went off so that remains unclear tonight. several officials say the two bombs that did go off included shrapnel to multiply the injuries. small pieces of metal. investigators are now studying video to see if they can see anyone actually placing packages at the point where is the bombs went off. they describe the devices as crude with thankfully less explosive power, brian, than might have been contained in a package of this size.

>> pete williams with the latest on what's known about this investigation. pete , thanks.