He only knows what you already know.

Castor leans forward. "I know their numbers, at least. There are about two dozen of them; well-armed, so dangerous, but not as large a **** as you might think, given the impact they've had."

You nod, frowning. "Yes, we'd heard that elsewhere. Is there anything else at all that you can tell us?"

Castor shakes his head, frowning. "No. I'm afraid that's all...well, all that's certain. And it's more than my job would be worth to share the uncertain things. Especially if you went and got killed based on my words. Sorry."

Your mind is racing. Clearly there is something else that he could tell you; he's just unwilling to do so. He might well get severely disciplined if it were ever discovered that he'd done so, of course; perhaps it's best to let that be. Still, you can't help but wonder if there's a way to convince him...