Whoever did the bombing had access to the premier locations, technical expertise and an understanding of the marathon’s dynamics. These Marathons require many volunteers for setup and logistics. Many of the volunteers are college students, some with engineering backgrounds. During the set up a culprit volunteer carrying bulky materials might go unnoticed. The investigators ought to looking at volunteers who like to read Inspire magazine. They should at least interview the volunteers and see if anyone noticed something unusual.
I agree with this.
From the new photos that show what appears to be a large bag (the device) sitting on the curb I have a hard time understanding how such a large bag could be just laying there unnoticed. I also find it difficult to understand how it being placed in such an obvious place would go unnoticed...Unless it was placed there by someone who would be deemed trustworthy or someone who is part of the scene and blends in well.
You would think a man dressed in black with his head covered placing a large package right by the road would not go unnoticed, but I have also observed how poor humans can be at situational awareness in crowded conditions where they are extremely distracted ( looking for loved ones finishing a race)
I really hope I’m wrong but I have a hunch it was placed there by someone who would appear to fit in... your volunteer hypothesis fits.