The FBI reportedly plans to keep the tool it used to crack a dead terrorist's iPhone a secret.

FOX's Rachel Sutherland reports from Washington:

The FBI doesn't have any plans to tell Apple anything about the tool it used to unlock Syed Farook's encrypted device. Sources familiar with the matter say the FBI knows how to use the tool, but agents aren't entirely sure how it works.

Yesterday, Director James Comey told a cyber security conference in Washington that the agency was still weighing the matter:

(Comey) "We are in the midst of trying to sort that out and that involves answering a key question, which is, what do we know about the vulnerability?"

The decision to withhold the information would leave Apple out of the loop on a vulnerability on some iPhones, which will likely anger privacy advocates.

In Washington, Rachel Sutherland, FOX News.