This is a photograph of a backpack allegedly used in the Boston marathon "boming". (Source: ABC News)
Notice the square white patch on the backpack, circled in above picture in red.
The FBI released this photo of the Tsarnaevs at the Boston marathon prior to the "bombing" (Source: Guardian)
Another picture of the Tsarnaevs at the Boston marathon (Source: Daily Mail)
At left is Tamerlan Tsarnaev, wearing a dark grey backpack. Center is Dzhokhar Tsarnaev, wearing a light grey backpack. Neither of the Tsarnaevs' backpacks match the one allegedly used in the bombing. At right is what appears to be a member of Craft International wearing a black backpack with a white patch that closely resembles the backpack allegedly used in the bombing.
Agents of Craft Intenational (private military/security firm) on the scene, fitted with earpieces