At the time, the gas was reported to be an unknown incapacitating agent called "Kolokol-1".

Before Khamel can kill her he is shot and killed by an unknown agent.

Or to put it another way: the unknown agent would have had to come on board when you and Muzzel did.

The nun's blood was precious beyond measure, since it contained the unknown hot agent.

And so you must also know that an unseen, unknown agent is at work against us.

But I do hope that you have at least dug up those unknown agents by now.

He has appointed an unknown agent in my place.

On the scientific level, first of all this disease is due to an unknown agent.

"What we have here is an unknown agent that causes death in humans and sea life less than one minute after contact."

For that I need a skilled, trustworthy, and unknown agent.