An anonymous TPM Reader gives us a look from the inside …
In your post, you ask “why wouldn’t [they] go back during the plot itself?” in regards to phone records.
I guarantee they did just that. I’ve worked for AT&T for close to 15 years now, spending most of my time in the network engineering side of the house. Call records on our network can be pulled for many, many months in the past. Getting these call records is probably one of the first things they did in the investigation so they probably weren’t part of this later
Verizon’s network is a little different than ours, despite this, they can probably see most of the same data we can. I can pull call records for a single number, which will give me phone calls, text messages, and internet data sessions. On phone calls & text messages, I can see the length of the call, the number of characters in a text message, the cell site the call/text was placed on. I can also see the same information for the receiving end of calls or texts.
I can also pull all phone calls/texts placed on a single site for any given time. I would imagine that in a city like Boston, with a fairly high cell site density, you could narrow down the sender/receiver of a call/text to a square mile (or less) fairly easily.
For the rest of the story: http://editors.talkingpointsmemo.com/archives/2013/06/from_the_inside.php