There's no word on who this third party is (hint: it wasn't Apple) but Christopher Soghoian, the principal technologist with the American Civil Liberties Union, suggests they could have been paid tens or hundreds of thousands of dollars for their help.
But how did they do it?
Digital forensics expert Bradley Schatz — an Adjunct Associate Professor at the Queensland University of Technology who also runs a consulting firm — says speculation has been rife in tech circles, with two possible theories proving to be the most popular.
1. They used a bug
Dr Schatz says the FBI could have taken the exploitation route — as in they loaded some foreign software onto the phone to let them in.
Close up of the lock screen on an iPhone with a message saying Try Again.
Photo: The FBI had no way of knowing the shooter's iPhone passcode. (ABC News)
Bootloader bugs were more common in the early days of the iPhone, and "allowed people to run software on the phone that Apple hadn't blessed".
Those bugs were mostly ironed out by the time the iPhone 4S hit the market, but Dr Schatz says it's possible there are still some floating around.
What's the problem with this method? The bugs are closely held secrets, Dr Schatz says.
"They are very valuable and people aren't prepared to widely advertise them."
2. They copied the chip
iPhone users can opt to erase their data after the wrong passcode is entered 10 times.
This was a big concern for the FBI, as they couldn't have been sure they would be able to try more than nine passcodes without wiping the iPhone's data.
But Dr Schatz says authorities could have copied the phone's flash storage chip so they could make as many attempts as they wanted to.
This is also known as NAND mirroring — with the NAND being the flash memory.
Tech blogger Johnathan Zdziarski — an expert in iOS forensics — likens this to "kind of like cheating at Super Mario Bros with a save-game, allowing you to play the same level over and over after you keep dying".
The problem with this method is that the chip has to be soldered off the phone's motherboard and then resoldered back on, which can cause heat damage, Dr Schatz says.
Another way to use the same method would have been to copy the chip onto a flash memory emulator — a piece of hardware that would pretend to be the memory chip.
This would remove the step of having to solder a new chip onto the motherboard every time.
FBI director James Comey appeared to dismiss this method, saying he'd heard about it but "it doesn't work" — a statement with which Mr Zdziarski disagrees.
What does this mean?
At the end of the day, even Mr Zdziarski has no idea how the FBI did it. "Your guess is as good as mine," he says.
However, there are implications for your own phone's security.
"What is certain, however, is that the only reason this was possible is because Farook chose to use a weak form of security on his iOS device — namely a numeric pin," Mr Zdziarski says.
"To protect your device against both a hardware and software attack, use an alphanumeric passcode."
But Dr Schatz says iPhone users should take some comfort in how long it has taken the FBI to resolve the San Bernardino issue.
"The fact that it's taken the FBI so long to come up with a solution to this for this particular phone reflects the degree of difficulty in breaking through the security that Apple's implemented."
