Only Apple knows, but I'm going to guess they *won't* do it. I suspect the FBI has a pretty good idea what is and what isn't possible, especially since Apple has otherwise been cooperating with them. Also the people who work for the FBI aren't idiots, and I bet this isn't the first crime they've investigated with an iPhone.

Furthermore, Apple's argument against breaking this particular phone seems to be that they think such an action would compromise *all* phones. Despite the popularity of this belief, which is grounded on suspicion more than fact, The [actual FBI order][1] asks for a firmware which:

  - is limited to just the device in question
  - does not need to leave Apple's facility

Specifically, the FBI **does not** ask for:

  - an exploit that could be applied to any phone
  - access to the exploitable firmware
  - unsupervised access to the exploited phone
  - access to Apple's code signing key

IANAL, but I bet such things are unlawful. Even if you want to believe the FBI is a malicious organization, they won't ask for such things in a court order.

Here's the relevant section of the order, with interesting parts highlighted:

> Apple's reasonable technical assistance shall accomplish the following three important functions: (1) it will bypass or disable the auto-erase function whether or not it has been enabled; (2) it will enable the FBI to submit passcodes to the SUBJECT DEVICE for testing electronically via the physical device port, Bluetooth, Wi-Fi, or other protocol available on the SUBJECT DEVICE and (3) it will ensure that when the FBI submits passcodes to the SUBJECT DEVICE, software running on the device will not purposefully introduce any additional delay between passcode attempts beyond what is incurred by Apple hardware. 
>
> Apple's reasonable technical assistance may include, but is not limited to: providing the FBI with a signed iPhone Software file, recovery bundle, or other Software Image File ("SIF") that can be loaded onto the SUBJECT DEVICE. The SIF will load and run from Random Access Memory and will not modify the iOS on the actual phone, the user data partition or system partition on the device's flash memory. **The SIF will be coded by Apple with a unique identifier of the phone so that the SIF would only load and execute on the SUBJECT DEVICE.** The SIF will be loaded via Device Firmware Upgrade ("DFU") mode, recovery mode, or other applicable mode available to the FBI. Once active on the SUBJECT DEVICE, the SIF will accomplish the three functions specified in paragraph 2. The SIF will be loaded on the SUBJECT DEVICE at either a government facility, **or alternatively, at an Apple facility; if the latter, Apple shall provide the government with remote access to the SUBJECT DEVICE through a computer allowing the government to conduct passcode recovery analysis.**

So apparently, the FBI believes Apple could write this compromised firmware such that it would work only on the specific phone the FBI needs to access. Since firmware must be signed by Apple to work, it should not be possible for the FBI or anyone else to modify this compromised firmware to work on another phone. If that were the case, the FBI could just modify the current firmware, without Apple's help. So this would seem to address the concern that any phone could be compromised.

The FBI is also willing to *give Apple the phone* so the firmware never even needs to be in the FBI's possession. That would seem to address the concern that the firmware would "fall into the wrong hands". Even if it did, it wouldn't be exploitable given the previous provision.

Given these provisions in the FBI's order that seem to specifically address the concerns in [Apple's letter][2], we can only guess what Apple's reason for refusing the order may be.

It's possible there's some technical detail that Apple isn't telling us, and of which the FBI isn't aware. Or perhaps the FBI decided to ask for something they already knew to be impossible.

It's also possible that Apple thinks this is good PR. Apple certainly has a financial interest in making the iPhone appear "unhackable even by the FBI". Apple may also be trying to leverage anti-government sentiment.

One could argue there is an *ethical* reason for not circumventing security measures on a phone, and that privacy is more important than all other concerns, even if that phone belonged to now dead terrorists, and the information on it could contain intelligence that could prevent further terrorist attacks.

One could also argue that relying on what is probably a 4-digit numeric key to encrypt data is hardly secure, and whether Apple helps or not, the FBI will get in that phone. They may just need to perform some more difficult physical attacks. Breaking a pathologically weak key by brute force is hardly a backdoor. It's the front door.

Maybe the courts will sort it out, maybe not. Probably we'll never know.

  [1]: https://assets.documentcloud.org/documents/2714005/SB-Shooter-Order-Compelling-Apple-Asst-iPhone.pdf
  [2]: http://www.apple.com/customer-letter/