A crowd shot taken at the Boston Marathon before the blast.
Investigators are trying to identify a white man in his 30s seen on video surveillance at both bombing locations before the blasts, sources told The Post.
Advertisement - story continues below
But the quality is poor, and they are seeking better-quality videos and photos of the same person.
Here’s a shot taken from above the crowd by Ben Levine at Deadspin.
The FBI said at this time there are no claims of responsibility.