They're made up of the fiercest fighters in the Taliban, al-Qaeda, and the Haqqani network. They were formed in 2009 to combat the increase in drone strikes that was killing off their leaders - largely because tips to the CIA on the whereabouts of the terrorists were increasing.

They're called the the Khurasan and their methods of identifying tipsters and executing them have spread terror throughout Pakistan and Afghanistan:

Made up mostly of Arabs and Uzbeks, the Khurasan, named after a province of an old Islamic empire, are a shadowy group of several hundred men who operate in North Waziristan, where Washington believes Haqqani network leaders are based.

CIA pilots, who remotely operate the drones, could step up their pursuit of the Haqqani network leaders after an attack on the U.S. mission in Kabul last month. That would likely prompt the Khurasan to become more ruthless, after capturing about 120 people they've accused of being spies since 2009. When suspected collaborators are caught, they are held in cells in a network of secret prisons across North Waziristan.

A committee of Khurasan clerics decides their fate. Most are declared guilty after what group members admit are "very, very harsh" interrogations.

"They are given electric shocks. If they don't help then an electric drill is used or the spies are forced to stand on electric heaters," said one Khurasan operative.

"Or nails are hammered into their bodies." Any attempt to intervene on behalf of people who are captured is risky. The Khurasan see that as collaboration with the enemy too and it is punishable by death.

Whenever someone is found guilty, the Khurasan make sure everyone knows about it.

"The spies are taken outside residential areas at night and shot dead. Their bodies are thrown on roadsides or squares in the town with a piece of paper warning others to refrain from this 'dirty' job of spying," said one operative.

The Nazis used a similar technique in occupied countries to discourage the underground. It actually worked gruesomely well - up to a point. Because the people they used as an example were usually innocent, the rest of the population were either cowed into submission or, in some cases, began to turn in underground leaders.

Even the SS and Gestapo would have a hard time matching the brutality of the Khurasan.