According to Apple, the password for the phone in question, was changed within 24/hrs of the FBI taking the phone into their possession. Meaning either the FBI is just trying to use this very public case to make an argument to force Apple to comply or they fucked up and locked the phone after having access to it.
Apple implies FBI screwup: iPhone Apple ID password changed in govt possession, backdoor unnecessary | 9to5Mac
pretty shady shit, they are doing. Added to this, Apple has offered 4 different ways to get them access to the data with out building a back door.
I don't really like Apple, but I give them props for standing their ground.