So you've all heard this by now. However it's pretty clear this isn't a tech issue, it's a political issue as the FBI actually have the back-ups from iCloud providing all the info they actually need and Apple have also provided multiple alternatives.
Anyway, here's Apple's filing to oppose the Order around that iPhone - personally I found it an interesting read, if a little dry at points.
https://www.documentcloud.org/docume...xus-IS300.html