Um, how about the fact that the older brother was throwing homemade explosives out the window a stolen car at the police, the owner of that bullet riddled stolen car said that the two brothers had bragged about their role in the marathon bombing, the younger brother ran the older brother over while trying to escape, a 7-11 near the MIT campus calls in and reports a robbery by the younger bro, the bros murder a campus cop at MIT minutes later, the bombs are a hallmark and recommended method of Al Qaeda, the older brother had videos on his youtube channel praising Al Qaeda and Chechen separatists, and the police say they've recovered 7 additional IED's at the two locations where the brothers have stayed....
The cops say the younger bro is on video dropping the backpack scant seconds before the bomb goes off in the spot it does. I'm assuming that will show up once he's caught. Based on the activity in the past 24 hours, I'd say it's a pretty high likelihood this isn't all a fabrication.