Federal Aviation Administration Advisory Circular: Aviation Security--Acceptance and Handling Procedures--Indirect Air Carrier Security [open pdf - 1MB]
"This Circular provides information and guidance which should be used by 'indirect air carriers' when accepting and handling property to be carried by 'air carriers' or by operator of any civil aircraft for transportation in air commerce. [...] While implementation of the security procedures set forth in Part 109 and recommended herein may increase the indirect air carrier's cost, it is believed that these costs will be offset to a certain extent through a possible reduction in pilferage problems; but an even greater benefit may accrue to the indirect air carrier or other shipper through assurance of a high level of protection against public endangerment through unauthorized introduction of explosives, incendiary devices, or other potentially hazardous materials into passenger carrying aircraft. FAA [Federal Aviation Administration] believes that prudent indirect air carriers and others with a sense of public responsibility, regardless of regulatory requirements, will want to institute these safeguards to further assure the safety and security of the air traveling public."
Federal Aviation Administration Advisory Circular (AC) No. 109-1
Federal Aviation Administration: http://www.airweb.faa.gov/