Precisely how those relationships play out, however, is often shrouded in secrecy.

A recently published audit by the Department of Justice has now offered a startling glimpse behind the scenes of those operations, revealing a world in which hundreds of millions of dollars have been doled out to thousands of informants over the last five years. Those informants include package delivery personnel, bus company employees, and Transportation Security Administration agents moonlighting as drug war spies — all operating with abysmal oversight and scant evidence of return on investment.

Published Thursday by the DOJ's Inspector General, the audit reports that from 2010 to 2015, the DEA boasted more than "18,000 active confidential sources assigned to its domestic offices, with over 9,000 of those sources receiving approximately $237 million in payments for information or services they provided." By comparison, the FBI is said to maintain a roster of some 15,000 informants.