The FBI is threatening to raise the stakes in its legal battle with Apple (APPL), suggesting it might demand access to the iPhone maker’s source code and secret electronic signature used to verify the legitimacy of its software updates.

Apple is challenging a recent court order that it write a special version of its iOS software with some security features disabled so the FBI would try to crack the passcode on an iPhone 5c that was used by Syed Rizwan Farook, the deceased San Bernardino shooter.

Seeking Apple’s source code would “raise the stakes considerably,” says Ed McAndrew, a Ballard Spahr attorney and former federal cybercrime prosecutor who with whom Apple cooperated on several cases. But the threat in the FBI’s brief may be “a little tongue in cheek,” he says. “The government’s point here is that they were trying to do this in a way that eases the burden on Apple.”

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