This excerpt from the book Silent Warfare: Understanding the World of Intelligence by Abram N. Shulsky and Gary J. Schmitt is a good primer on the basic types and sources of human intelligence, as well as the potential problems in relying on human sources.

Several staff statements issued by the 9/11 commission have addressed the intelligence community's difficulties in obtaining human intelligence prior to the Sept. 11 attacks:

In this February 1998 article for The Atlantic Monthly, a pseudonymous former CIA case officer assails the case officers in the agency's Directorate of Operations (the official name of the clandestine service, also known as the DO) for "debasing American espionage." He writes that the DO "has for years been running an espionage charade in most countries, deceiving itself and others about the value of its recruited agents and intelligence production" and warns that reform may be impossible. [The Atlantic Monthly, February 1998]

Shortly before the Sept. 11 attacks, The Atlantic Monthly published this prescient article by former CIA operative Reuel Marc Gerecht in which he argues that the U.S. intelligence community did not have the capability to gather human intelligence on Osama bin Laden or Al Qaeda. "Unless one of bin Laden's foot soldiers walks through the door of a U.S. consulate or embassy, the odds that a CIA counterterrorist officer will ever see one are extremely poor," he wrote. [The Atlantic Monthly, July/August 2001]

In this article for Foreign Affairs, Richard K. Betts, director of Columbia University's Institute of War and Peace Studies, argues that some of the reforms proposed in the wake of the Sept. 11 attacks may make things worse. On the topic of human intelligence, he writes, "Human intelligence is key because the essence of the terrorist threat is the capacity to conspire." However, he warns, "Building up human intelligence networks worldwide is a long-term project. It inevitably spawns concern about waste (many such networks will never produce anything useful), deception, (human sources are widely distrusted), and complicity with murderous characters... These are prices that can be borne politically in the present atmosphere of crisis. If the sense of crisis abates, however, commitment to the long-term project could falter." [Foreign Affairs, January/February 2002]

Bruce Hoffman writes, "Indeed, as is now constantly said, success in the struggle against Osama bin Laden and his minions will depend on good intelligence. But the experiences of other countries, fighting similar conflicts against similar enemies, suggest that Americans still do not appreciate the enormously difficult -- and morally complex -- problem that the imperative to gather 'good intelligence' entails." [The Atlantic Monthly, January 2002]