It turns out that the FBI really did find a third party solution to help them try to unlock the San Bernardino shooter's iPhone without involving Apple. According to the latest reports, the Department of Justice just signed a $15,000 contract with an Israeli firm called Cellebrite.
Cellebrite is a company which specializes in advanced mobile forensics data solutions to aid in investigations, corporate security, law enforcement and more. Here's a quote from their website describing their mission and services:
"Cellebrite mobile forensics solutions give access to and unlock the intelligence of mobile data sources to extend investigative capabilities, accelerate investigations, unify investigative teams and produce solid evidence. Cellebrite's range of mobile forensic products, the UFED Series, enable the bit-for-bit extraction and in-depth decoding and analysis of data from thousands of mobile devices, including feature phones, smartphones, portable GPS devices, tablets and phones manufactured with Chinese chipsets."
The primary means that Cellebrite is likely to take in order to "hack" the shooter's iPhone was described by UCLA technology fellow Daniel Kahn Gillmor. He believes they will use a technique called NAND mirroring.
This method copies the part of the phone's internal memory which counts the number of passcode attempts entered. Cellebrite can then continually restore the copy, so the FBI can bypass the limit passcode guesses which can normally be entered before the device is bricked.