Um, maybe they just looked at Twitter and Facebook? It's public.
How many FBI agents does it take to go through a square mile of those tweets and FBs? That’s not done by humans in two days.
There has to be a ‘puter some where that can scan for the time period, this one was +/-24 hours from the explosion on Monday, looking for key words, who’s texting etc. with who. Then it analyses the frequencies of the connecting posts to the same people etc.
Nah, this is big bro using the tools they built it for...