The FBI on Thursday threatened to raise the stakes in its legal battle with Apple (AAPL), 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.

The FBI currently is seeking to force Apple to write a special version of its iOS software with some security features disabled so that the bureau can try to crack the passcode on an iPhone 5C used by deceased San Bernardino shooter Syed Rizwan Farook. Apple is challenging the order issued by U.S. Magistrate Judge Sheri Pym last month.

But in a court filing on Thursday, the FBI said that if can't require Apple to create the weakened software, it may demand access to what it described as Apple's "crown jewels" instead. Source code is the list of programming code instructions used to create the software that runs the iPhone. The code controls everything from the background colors on the screen to the most critical security protections the phone has. Apple's secret signature is a digital "key" required to update software on all iPhones.

If the FBI got access to those two items, the bureau, or outside programming experts it hired, could try to write the security-weakened version of iOS and install it onto Farook's iPhone without Apple's assistance. But Apple would be likely to fight even harder to keep its source code and digital signature out of the government's hands.

"The FBI itself cannot modify the software on Farook's iPhone without access to the source code and Apple's private electronic signature," the agency wrote in its latest filing. "The government did not seek to compel Apple to turn those over because it believed such a request would be less palatable to Apple. If Apple would prefer that course, however, that may provide an alternative that requires less labor by Apple programmers."

Apple is so far refusing to write the weakened version of its software. In its filing challenging the request, Apple argued that the court lacked proper authority to order the software and that creating the weakened version would ultimately weaken the security of all iPhone users.

Seeking Apple’s source code would “raise the stakes considerably,” says Ed McAndrew, a lawyer at Ballard Spahr and a former federal cybercrime prosecutor who sought and received help from Apple on many 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 way that eases the burden on Apple.”

Overall, the 35-page FBI filing sought to portray its request for Apple to write the weakened iOS software as well within the bounds of prior court precedents. The filing also accused Apple of making “false” arguments and purposely raising technological barriers to block law enforcement access to data on iPhones.

Farook and his wife, Tashfeen Malik, shot and killed 14 people on December 2 at the Inland Regional Center in San Bernardino. Farook’s iPhone 5C, issued by his employer, the San Bernardino Department of Public Health, was found in the back of a car used by his mother. The shooters destroyed two other phones they had used on the day of the attack.

The FBI has said that the phone is locked with a passcode. Apple’s current iPhone software forces a lengthy delay after several incorrect guesses and can be set to erase the contents of a phone after 10 incorrect  guesses. So the FBI wants Apple to write a special version of its software without the delay and auto-erase features and that allows the bureau to input passcode guesses rapidly from an attached computer instead of slowed keyed in by hand one by one on the phone’s lock screen.