What is the name for the person who accompanies an airplane flight in case of hijacking crisis?

So, i have read about this sometime ago. The article was saying about how most flights nowadays will have a person, who is skilled at handling and communicating with hijackers, on board dressed up as civilians of course. So, on each and every flight, there will be one of this type of people to accompany the flight in case of a hijacking crisis. Something like "chaperoning" the flight? Does anyone knows what's the term or name of this profession? I'm pretty sure there's a name for it but i just couldn't remember what exactly is the name. Some help here would be greatly appreciated. Thanks!