The FBI says the solution it found to accessing data on the San Bernadino shooter’s iPhone won’t work for most later models. It also confirmed that it paid money for the solution.

FBI director James Comey said the bureau is still weighing up whether to hand over details of the method it used to allow it to brute force the iPhone 5c password. Other reports say they will brief two senators about how they did it.

Those senators, Dianne Feinstein and Richard Burr, are working on a bill that would make it mandatory for encrypted data to be accessible by the government if a judge so orders. However, the White House is reportedly refusing to give any public backing to the bill.

Comey also confirmed the method will only work on the iPhone 5c running iOS9. That’s led some commentators to deduce the distinction may be that later models use a 64-bit chip rather than a 32-bit chip in the iPhone 5c. In that case it could well be that the FBI’s method isn’t fundamentally impossible to use in the later handsets, but rather completely impractical in the real world.

Exactly who provided the method is also under wraps, with Comey saying “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.”

Since agreeing to try its method to help prosecutors in an Arkansas murder case, the FBI has been flooded with requests from local police to help access phones. It may be that officials have decided to confirm the limited scope of the method to stave off some of these requests.