It possibly came into being after the civil war and then president Ulysees Grant. The need arises for clandestine yet forceful intervention into reality to preserve the existence of the democratic nation.
The psychologists call for a person, not sadistic, not unduly violent or aligned with fringe political groups. He or she must feel an undying affection for their country no matter where it leads them. He or she might be burdened by guilt, guilt that impinges upon consciousness about a crime committed that cannot be righted by any action, no matter how violent or money endowed that became in their early existence. Now they are empowered by righteousness and a sense to avenge their evil transgression. They are now and they exist in a mortal mind, in a high place that might die very soon. They are not real and they will never be.
They are not paid because a paycheck is part of an account that ends up on a computer system. Where they go is a bag of unmarked bills, enough to get them through an assignment and all the necessary gear to carry out the imperative. A key arrives in a manilla envelope to a home where he or she leads a common non-descript existence. They could be accountants or insurance men or real estate agents, or maybe even a salesperson, and they have a family with children. In the manilla envelope, the key has a tag with an address and a date and the person wherever they live, calls in sick, or whatever and then leaves an goes to an unknown location in a circuitous route immediately. At the destination, usually a motel or a pay-share, they find a cache of bills, and any necessary ordinance they require to carry out their mission. Instructions are written, on disposable paper and surely, they flush down a toilet. The list is only to be read once and then destroyed. He or she then carries out a shooting, or a robbery, or a hacking at a server, or an impersonation at an important place to carry out a distraction for an event that is about to occur.
After the agent complies with the written order signed by an authority, they destroy everything and take the quickest way home without attracting attention. He or she then take a shower and go back to a humdrum existence. The mode of transportation is the key, and they do not exist and they never happened. They get to keep anything and everything they can steal on the way too and from and use the infrastructure provided to commit crimes. They are not on the books, and they do not exist, the memory of them resides in the mind of the person that sent them, who configured them, and that person has to ultimately die.
Take some time to go to Arlington, and kneel at the tomb, and say a prayer, and then leave forever. Without these men and women, the bad guys ultimately win and without them our great nation would not exist. Under God we trust, and our nation is delivered by them for another day. Ask for the luck of the Irish, and Gods speed, and remember the Lord helps people that help themselves.