Apple's refusal to build a crippled iOS that will help the FBI unlock a killer's iPhone goes far beyond the executive suite. Some of Cupertino's own engineers are refusing to work on the operating system in case they are forced to aid the US government.

Register sources familiar with the matter told us that, since the start of the iGiant legal showdown with the Feds in early February, Apple developers have been reluctant to work on certain iOS projects and engineering assignments for fear of being summoned by the FBI and forced to break their own product under legal duress.

The opposition to the FBI's demand runs so deep that some Apple engineers have even threatened to walk away from the company: The New York Times cited "current and former" employees who said members of Apple's engineering teams are prepared to quit if they are ordered to weaken iOS's security mechanisms.

Publicly, Apple executives have showed no signs of backing down. Led by CEO Tim Cook, Apple has steadfastly refused to provide the FBI with a custom-built and cryptographically signed version of iOS that will let investigators guess the PIN code to San Bernardino shooter Syed Farook's iPhone 5C without the device wiping itself after too many wrong attempts. The special firmware will be installed during boot up, and only Apple can produce code that will be trusted by the hardware.

The Feds say they need evidence of Farook's terrorist links from the handset, even though they already have copies of his iCloud backups, the 5C was Farook's work phone as a local government employee, and he destroyed his other devices.

Most recently, Cook said that "according to the government, short of kidnapping or breaking an express law, the courts can order private parties to do virtually anything the Justice Department and FBI can dream up," and that the nation's founding fathers "would be appalled" at the FBI's demands.

The FBI has said that it is only seeking to break into Farook's smartmobe, and not all iPhones. However, it has been noted that prosecutors have already made similar requests for device access in other criminal cases, and will likely use the outcome of the Farook case as a precedent to demand that other iPhones be unlocked by Apple at the behest of police and g-men.

Meanwhile, security researchers, and whistleblower Edward Snowden, believe that the FBI already has the means to access Farook's data and could likely unlock the phone without Apple's help, using either third-party engineers or tools developed by the NSA. ®