Terrorists work in cells there is no connection with Farook and the other terrorists over in Belgium, they are terrorists and basically believe in the same stupid ideology and basically follow their mantra and that is why they all seem connected.
The big problem is the news, it's one to send the messages to the intended targets and their own followers. You get one that start and you hear it in the news, you catch the guy(s) and few days later there is another attack, it just daisy chain. The more the media will provide details the worse it will get.
The terrorist don't use cell phone except at the very last moment when they want to do their attacks they know their cell phones communications are tracked, they use burner that is not even encrypted because in the last minute it's just too late to catch them anyway. Burner cell should be illegal in the first place. Every cell phone should be registered to somebody period, is it doable I don't know.
This has been proved that after the Snowden incident that they could not catch a lot of peoples with cell phone, they did don't get me wrong but to pinpoint terrorists it's mostly useless for those attacks.
So this whole thing about giving away our privacy and putting a back door on cell phones does not render security but less.
As for the access to authorities to have access to cell network well they need to have a way at least to catch the criminals even though for terrorist seem pointless but there is dumb wannabe terrorist so that is at least that.
All in all no backdoor to the cell phones if they can do it the hard way it's one thing because only some company and authority have the mean to make it happen but a backdoor anybody could have access to any cell phones just a matter of time.