Defense lawyer's dream: some shmuck runs unknown SW to gather evidence about a suspect with a completely clueless LEO watching. Judge: the evidence is inadmissable, the conviction is vacated.
If you are suggesting the phone contents could aid further investigations that is different. But that data can be gathered other ways. Start with all the people who ever went to his mosque.
Don’t terrorists cell phones reach further than the nearest mosque?
In reality, they just want to connect any dots, which may be gone by now but a clue is a clue.
In actuality, the FBI would need/want what info that could be left on that phone? The provider has already, I am sure, turned over all incoming/outgoing calls as well as texts. So a call/contact listing?