I can verify from personal acquaintances that there was indeed some sort of event that occurred. (I know someone who already crossed the finish line a minutes or 2 before this. She heard all the commotion & just continued running to get the F out of there. I also know someone else who was still miles behind.) I can also verify that the whole greater Boston area was locked down for a few days after while they supposedly were pursuing the brothers.

Please note, I am NOT contradicting what you are saying. I am just trying to add other things I know to be true. One thing that hit me odd back then was that, when the whole metropolitan area was in lockdown while authorities were in pursuit of the brothers who by then were in Watertown. I noted that I heard police sirens going off in exactly the OPPOSITE direction of where Watertown was for me from where I was at the time.

Also, an acquaintance of mine knew the mother of the brothers. She was once the cleaning lady where my acquaintance once worked but she knew her well enough as to wonder if she should call her in condolences of her son. My acquaintance said that the mother wasn't a very observant Muslim when she knew her & never wore the scarf. She also had brought the younger son into work with her at times. I asked what she thought of him (since he was supposedly a terrorist) She said he was a very well behaved kid.