LONDON A Jordanian double agent's killing of CIA employees in Afghanistan shows the complex espionage challenge posed by Islamist militants for the West.
Following are some of the risks and rewards double agents and defectors pose for spy services, an aspect of espionage that former CIA counter-intelligence chief James Jesus Angleton once called a "wilderness of mirrors".
An intelligence officer recruiting a double agent is starting a relationship with someone risking at least a long prison term and probably execution for betraying their own organisation.
The first questions the officer must answer include, "Is this betrayal real?" and "If so, how do I protect this person from being discovered?"
The first question is especially problematic as the officer will have no standard means of verification -- a check can hardly be run with the agent's parent organisation, be that a conventional intelligence service or a militant group.
Psychology will play a role in assessing the double agent and their motivations, which may include money, ideology, coercion or personality quirks such as ego or self-importance.
If the officer is blackmailing the agent, it may not be clear if the target is succumbing genuinely to the pressure.
When an officer is cultivating a double agent the supreme challenge is to win their trust: The agent is putting his or her life in the officer's hands.
But communicating a convivial message of welcome may be difficult if personal contact is intermittent, indirect or impossible due to the absolute need for discretion and secrecy.
Even if personal contact is possible, trust can be destroyed if the potential double agent, often suffering high stress, are given even a faint impression that they are under suspicion.
The value of double agents
Double agents are valuable because they can shed light on the opposition's strengths, weaknesses and intentions. He or she may be in a position to neutralise these ambitions.
The double agent can be used to confuse the enemy or manipulate it.
The intelligence officer's success is advanced by the success of his or her agent. This reality entails a danger. Seeking yet further success, the officer may overlook suspect behaviour by the agent for fear of driving the agent away or ending the run of achievement.
"Recruiting an agent from another service, whether a rival service or a so-called friendly one is the acme of every intelligence officer's career. He or she will do anything to achieve that," wrote espionage expert Phillip Knightley.
Value of defectors
Defectors have been described as the lifeblood of Western intelligence services.
The defector brings up to date knowledge of the opposition. If the defector is well placed, he or she may bring clues to the identity of opposition agents or "moles" in the organisation to which he is defecting. The defector can bring an assessment of what the opposition knows about the intelligence service.
In the Cold War, Western intelligence spent time money and energy trying to identity possible defectors and then trying to persuade or blackmail agents into coming over.
Dangers of defectors
Defectors are dangerous because of the need to decide whether they are genuine or acting under orders as part of a plan to plant a mole or propagate disinformation.
A lesson from the Cold War?
A defector who arrives unexpectedly, who "walks in", is often regarded with suspicion.
In the Cold War Western spies preferred a defector they had "targeted themselves, worked on for a long time and who comes reluctantly, preferably kicking and screaming," wrote Knightley.
"The rationale is simple and cynical: If he can be "turned" once, he can be "turned" again."
(Sources: Former U.S. intelligence officer Robert Ayers, Phillip Knightley's The Second Oldest Profession, Chris Early interview with former KGB spy Boris Solomatin at www.trutv.com/library/crime/)
(For more Reuters coverage of Afghanistan and Pakistan, click here)
(Reporting by William Maclean, editing by Dominic Evans)