"The FBI is very good at keeping secrets, and the people we bought this from, I know a fair amount about them, and I have a high degree of confidence that they are very good at protecting it and their motivations align with ours," Comey said. The FBI hasn't said who provided the hacking tool.

Discussions are continuing about whether to provide Apple with details about how the hack was carried out, Comey said. "That's an interesting conversation because we tell Apple and they're going to fix it, and then we're back where we started from," he said. "As silly as that may sound, we may end up there. We just haven't' decided yet."