crime vs. phones
In the case of the San Berdo shooter, there were questions about affiliation with terrorist groups and after investigators screwed up and reset passwords for the associated iCloud account to the phone they were lost to figure out how to use any other avenue to investigate. I guess they become so used to gleaning data from phones that if they can't, they're stuck.
In the case of this shooting, there doesn't seem to be any connection to a network of nutjobs, this whacko was most likely a solo act. If there is a question, why can't the FBI send a query to the NSA to have somebody look and listen to this guys cell phone recordings to see if there is anything there? Does the FBI need admissible evidence for something or are they just fishing? Obviously, they can't outright admit that the NSA is recording everybody's texts, meta data and voice calls even if it's widely suspected.