For what it is worth, the person in the surveillance video:

The person of interest shown on a surveillance video setting down a bag
near the site of the second explosion at the Boston Marathon is
described as a young white male wearing a black jacket, a grey hooded
sweatshirt and a white or off-white baseball cap backwards. He is 6 feet
or 6-feet-2-inches tall with a medium build.

The man appeared to be alone and was talking on a cell phone at the time
he set down a bag. When the first explosive device went off, he took
off. Moments later, a second explosion happened right where he had been
standing.

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I hope the description is accurate.