Jonathan Zdziarski, writing on his security blog:

FBI acknowledged today that there “appears” to be an alternative way into Farook’s iPhone 5c – something that experts have been shouting for weeks now; in fact, we’ve been saying there are several viable methods.

And:

We do know that the FBI frequently works with a number of contracted external forensics and data recovery labs, especially a handful at the top. The likelihood here is that a third party contractor, such as one of these forensics or data recovery firms, has devised a method and notified FBI of their findings.

And:

The leading theory at present, based on all of this, is that an external forensics company, with hardware capabilities, is likely copying the NAND storage off the chip and frequently re-copying all or part of the chip’s contents back to the device in order to brute force the pin – and may or may not also be using older gear from iOS 8 techniques to do it. The two weeks the FBI has asked for are not to develop this technique (it’s most likely already been developed, if FBI is willing to vacate a hearing over it), but rather to demonstrate, and possibly sell, the technique to FBI by means of a field test on some demo units.

All this is speculation, but very well reasoned speculation.