It might not have been bitten, it might have just snapped.

The main suspect would be a shark or ray. But when the line is under load pretty much any large fish. Even a rock could cut the steel trace when under load.

Steel trace still has a breaking strain limit just like normal fishing line. If your rod bent fully over, then you would have been close or even past the breaking limit.

Two things I can come up with; One. A croc. or gator. second the steel wire doubled back on it self forming a loop. This when pulled tight will cause the wire to snap. inspecting the end should reveal all.
A slight kink will mean a loop. a jagged edge a tooth mark. 
Enjoy your forensic study.:'( 

Interesting!
Based on my experience with situations like this (snapped steel wires) it can be either a shark or a barracuda. 
Big Barras are capable of cutting through steel wire. 
However, from the behaviour you are describing it sounds more like something it also might be an amberjack that you have hooked, which then dragged you on a rock and that might have snapped your steel wire, even though, regardless of what lots of people say, you can assume safely that steel wire does not break that easily when shear force is acting on it from a rock! 

i think its a Barracuda!

Is the place you were fishing famous for attracting Barras?

Methos