The FBI says that, while it has unlocked the iPhone 5C, it cannot replicate the hack on the iPhone 5S or more recent devices.
"It's a bit of a technological corner case, because the world has moved on to sixes," FBI director James Comey said during a keynote at Kenyon College last night. "This doesn't work on sixes, doesn't work on a 5s. So we have a tool that works on a narrow slice of phones."
Comey added that the FBI is still debating whether or not to tell Apple about the vulnerability—though given the specificity of the hack, it seems unlikely they would disclose that information.