The air cargo industry consists of a complex distribution network linking manufacturers and shippers to freight forwarders, off-airport freight consolidators, and airport sorting and cargo handling facilities where shipments are loaded on and unloaded from aircraft. While only a small fraction of cargo shipments travels by air, items shipped on aircraft generally consist of time-sensitive and high-value commodities. By weight, air freight comprised only 0.05% of all domestic and international shipments to U.S. points in 2015, but these accounted for 6.1% of the value of freight shipments. Common examples of air cargo include high-value machine parts and manufacturing equipment, electronic components for manufactured goods, consumer electronics, jewelry, and perishable items such as flowers, fruits, and fresh fish. Specialized freight that requires specific handling—such as unique scientific instruments, highly specialized tools and equipment, and even thoroughbred horses—is also transported as air cargo. Most outbound air cargo packages are consolidated at off-airport facilities and arrive at airports on bulk pallets or in special containers known as unit load devices. It is estimated that about 75% of all air cargo by weight travels on bulk pallets. Typically, shippers have no foreknowledge of the route or aircraft by which a particular shipment will be transported. Freight forwarders and airlines make such determinations, using logistics software, databases, and computerized flight schedules to optimize the flow of air cargo. Both domestic and international air cargo movements generally rely on a hub-and-spoke network of airports to link origins and destinations. Most international air cargo that enters the U.S. transits through large hub facilities in Europe and Asia. While business and consumer demand for the fast and efficient shipment of goods fueled rapid growth in the air cargo industry in the 1980s, 1990s, and early 2000s, the industry saw little growth over the past decade, and it was not until 2017 that air cargo shipments surpassed 2007 levels reached prior to the 2007-2009 recession. Through October 2017, however, air cargo activity increased more than 10% from 2016 levels, the largest year-over-year growth in more than a decade, with the largest growth seen among international shipments to and from the United States, particularly shipments from Europe. The Federal Aviation Administration (FAA) forecasts that steady U.S. and world economic growth will drive more modest annual increases of about 3% in air cargo shipments over the next two decades, while Boeing notes that e-commerce could spur additional demand for worldwide air freight shipments. About 22 billion pounds of freight cargo were shipped on domestic flights in 2016. Of this, FedEx transported about 11.6 billion pounds, while UPS carried about 6.3 billion pounds. Collectively, these two carriers transported about 81% of all domestic air cargo in 2016, and were by far the largest two operators in the U.S. air cargo industry. Additionally, in 2016, approximately 19 billion pounds of international air cargo were transported to and from the United States. While FedEx and UPS were the largest carriers by weight carried, combined they transported only about 13.2% of international air cargo to and from the United States. Their comparatively smaller role in the international sector reflects a greater number and diversity of air carriers that transport cargo that originates overseas, including a number of relatively small operators and much greater reliance on passenger air carriers. Passenger aircraft play a greater role in transporting air cargo internationally than within the United States. On international routes to and from the United States, about 38.5% of air cargo by weight is transported on passenger aircraft. In the domestic market, on the other hand, only 5.5% of air cargo is carried in the bellies of passenger planes. This characteristic is of particular interest with respect to potential security vulnerabilities, as cargo shipments could provide a means of placing explosive devices aboard international passenger flights destined for the United States. Security of air cargo shipments within the United States and international shipments to and from the United States is regulated by the Transportation Security Administration (TSA). Additionally, international air cargo shipments entering the United States are regulated by U.S. Customs and Border Protection (CBP). Over the past decade, particular attention has been devoted to overseeing the security of cargo loaded on passenger aircraft. In addition to domestic operations at about 450 commercial service airports, nonstop flights from about 140 foreign airports in more than 80 countries transport cargo to the United States. TSA points out that a large proportion of the cargo shipped to the United States does not originate at these last-point-of-departure airports and is often carried on multiple legs using both passenger and all-cargo aircraft before being loaded onto a U.S.-bound flight. Addressing potential threats requires security measures along the cargo supply chain, with particular attention to ensuring that risk-based evaluation and physical screening of shipments are conducted using techniques and technologies that comply with statutory and regulatory requirements. Despite concern over the potential use of air cargo to introduce an explosive device aboard a passenger aircraft, no such attack has ever been successfully carried out. Concern over this type of threat is largely predicated on the belief that more stringent measures to screen passengers and baggage may cause terrorists to consider the detection of an explosive device in air cargo to be less likely. In response to these concerns, U.S. policy has shifted from risk-based approaches for vetting cargo placed on passenger aircraft to mandatory 100% physical screening requirements for all air cargo that travels within the United States or to the United States aboard passenger aircraft. In contrast, all-cargo aircraft have generally been considered less attractive targets for terrorist bombings, and TSA has continued to apply risk-based measures for vetting shipments placed on all-cargo aircraft. Prior to 2010, one of the few known plots to place a bomb aboard U.S.-bound cargo planes in the mid-1990s was broken up and did not result in significant policy changes. Historically, security measures surrounding all-cargo operations have focused on the threat of hijackings, particularly hijackings that could result in a suicide attack using the aircraft as a weapon of mass destruction. The potential for such an attack was illustrated in a dramatic incident that occurred on April 7, 1994, several years prior to the use of hijacked aircraft to attack the World Trade Center and the Pentagon on September 11, 2001. In the 1994 incident, an off-duty FedEx flight engineer attempted to hijack a FedEx DC-10 aircraft and crash it into the company's Memphis, TN, headquarters. The hijacker boarded the airplane in Memphis under the guise of seeking free transportation (a practice known in the industry as deadheading) to San Jose, CA. His only luggage was a guitar case that concealed hammers, mallets, a knife, and a spear gun. At the time, there was no federal requirement or company procedure to screen personnel or personal baggage carried aboard cargo aircraft. The three flight crew members thwarted the hijacker's attempt to take over the airplane and made a successful emergency landing in Memphis despite all three crew members on board having sustained serious and life-threatening injuries. The threat of explosives has been a long-standing concern for cargo loaded on passenger aircraft. Several incidents have shown that terrorists may seek to target U.S.-bound air cargo shipments, including shipments placed on all-cargo aircraft, by exploiting weaknesses in air cargo security overseas. On October 29, 2010, intelligence and law enforcement agencies in Dubai, United Arab Emirates, and in the United Kingdom discovered explosive devices concealed in packages shipped as air cargo bound for the United States. According to media reports, the explosives were not detected by initial screening, but were discovered upon reexamination after authorities received a tipoff from a member of the Al Qaeda terrorist organization who had turned himself in to officials in Saudi Arabia prior to the incident. The first shipment was located among parcels offloaded from a UPS cargo flight that arrived at East Midlands Airport, England, after having transited on all-cargo flights from Sana'a, Yemen, to Dubai; from Dubai to Cologne/Bonn, Germany; and from Cologne/Bonn to East Midlands. The package was scheduled to continue via Philadelphia, PA, to Chicago, IL. The second parcel was discovered on a Qatar Airways passenger jet in Dubai after having flown from Sana'a to Doha, Qatar, and on to Dubai aboard two passenger flights. The package was scheduled to continue on a U.S.-bound FedEx all-cargo aircraft to Chicago via Newark, NJ. Authorities in the United Kingdom surmised that the explosives were probably intended to detonate in flight, possibly over a U.S. city, and were capable of bringing down the aircraft. Both explosive devices contained pentaerythiritol tetranitrate (PETN), a powerful explosive that is difficult to detect, in quantities considered sufficient by explosives experts to cause catastrophic damage to a large airliner if detonated during flight. Their discovery prompted a rethinking of the generally accepted belief that bombing an all-cargo aircraft is considerably less attractive to terrorists than bombing a passenger plane. Much remains unknown about the motives and objectives behind these incidents. The possibility that the terrorists intended to bring about more restrictive regulations and thus cause widespread economic damage to the air cargo industry cannot be excluded. The details of the October 2010 plot highlighted a number of specific challenges to securing air cargo. First, the explosives were only discovered after a tip from intelligence sources, but even then proved difficult to detect. Reportedly, initial inspections by British authorities at Midlands Airport using canines, explosives detection equipment, and visual searches did not detect the explosives. This raised concerns that well-concealed explosives could be difficult to detect with routine methods of physically screening cargo. Second, the nature of the shipments, given that Yemen is not a usual source of printer cartridges supplied to the United States, raised questions regarding the implementation and effectiveness of risk-based targeting methods to identify suspicious cargo.