Chiglet, yeah exactly. The other poster mentioned turning on "the TCAS with mode C", i was pointing out the interceptor involved would have an ATC Mode 3 transponder but no TCAS.
As for ATC being able to see them, yes they would be able to, i guess the civil controllers would see the Primary return, the Mil controllers (Fighter controllers or GCI) will be able to monitor the aircraft through it's other Transponder Modes.
Here's one of mine:
