How could they possibly have known that without unlocking the phone?
Umm, no - no they aren't. Not even close. The terrorists personal phones were destroyed before the FBI could recover them, this was just a 'work phone'.
Don't you think it's likely that a) there is a reason they destroyed their personal phones, and b) if they were going to communicate with other actors they'd be more likely to use the phone that's completely under their control?
You do know that they destroyed their personal phones, right? And that it's been months now, wasted bickering with Apple even if there would have been any leads.
> How could they possibly have known that without unlocking the phone?
Smart money says they already unlocked it with an exploit and knew there was nothing, then played legal games with Apple until they started losing, and to exit gracefully they accepted the offer of one of the many firms begging to extract data from the phone for them.
They have the NSA's phone number. They were playing helpless for a reason.