In the crowdsource images, there are so far three possible 'guy nearby with backpack, then spotted walking away apparently without backpack'.
Keep in mind that there may be multiple people involved.
Dammit I hate 'slideshow' pages like http://www.reuters.com/article/slideshow/idUSBRE93F06T201304... Such a pain saving the images. Reuters, please just post a zipfile somewhere of the original full resolution images, please, please.
Some more good images at http://cryptome.org/2013-info/04/boston-bombs/boston-bombs.h... http://cryptome.org/2013-info/04/boston-bombs-02/boston-bomb...