The technique used to recently crack an iPhone 5c will not work on the iPhone 5s and later models, said FBI director James Comey.

The FBI claimed it successfully accessed data on a smartphone belonging to one of the shooters in a mass killing in San Bernardino. Apple’s reluctance to participate in the work sparked an industry-wide debate into user privacy versus government demands.

The fact that the workaround is limited to the aging iPhone 5c is significant, given the encryption standoff between Apple and the US government which was broken by the FBI hack.

If it turns out to have a limited impact then we are more likely to see standoff reruns in the case of later model iPhones the government wishes to access.

The mystery iPhone help was thought to have come from Israeli firm Cellebrite, although this has not been confirmed by the bureau or the firm.

“It’s a bit of a technological corner case, because the world has moved on to [iPhone] 6,” said Comey, as reported by The Verge. “This doesn’t work on the 6, doesn’t work on a 5s. So we have a tool that works on a narrow slice of phones,” he added.

“I can never be completely confident, but I’m pretty confident about that,” Comey concluded.

His comments were made in response to a question during an appearance at Kenyon University.

However, the director did not elaborate on why more recent models are trickier to access. The Verge reckoned it’s likely to do with the Secure Enclave protections that were implemented with the iPhone 5s’ A7 chip, which is present in all subsequent iPhones.