"For the reasons discussed above, the FBI cannot itself modify the software on the San Bernardino shooter's iPhone without access to the source code and Apple's private electronic signature. 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."
As Fortune's Philip-Elmer DeWitt rightfully pointed out, that's a classic police threat. "We can do this [the] easy way or the hard way. Give us the little thing we're asking for -- a way to bypass your security software -- or we'll take [the] whole thing: your crown jewels and the royal seal too," DeWitt wrote. "With Apple's source code, the FBI could, in theory, create its own version of iOS with the security features stripped out. Stamped with Apple's electronic signature, the Bureau's versions of iOS could pass for the real thing," he added.