The nation's air, land, and marine transportation systems are designed for accessibility and efficiency, two characteristics that make them vulnerable to attack. The difficulty and cost of protecting the transportation sector from attack raises a core question for policymakers: how much effort and resources to put toward protecting potential targets versus pursuing and fighting terrorists. While hardening the transportation sector against terrorist attack is difficult, measures can be taken to deter terrorists. The focus of debate is how best to construct and finance a system of deterrence, protection, and response that effectively reduces the possibility and consequences of terrorist attacks without unduly interfering with travel, commerce, and civil liberties. For all modes of transportation, one can identify four principal policy objectives that would support a system of deterrence and protection: (1) ensuring the trustworthiness of the passengers and the cargo flowing through the system; (2) ensuring the trustworthiness of the transportation workers who operate and service the vehicles, assist the passengers, or handle the cargo; (3) ensuring the trustworthiness of the private companies that operate in the system, such as the carriers, shippers, agents, and brokers; and (4) establishing a perimeter of security around transportation facilities and vehicles in operation. The first three policy objectives are concerned with preventing an attack from within a transportation system, such as occurred on September 11, 2001. The concern is that attackers could once again disguise themselves as legitimate passengers (or shippers or workers) to get in position to launch an attack. The fourth policy objective is concerned with preventing an attack from outside a transportation system. For instance, terrorists could ram a bomb-laden speedboat into an oil tanker, as was done in October 2002 to the French oil tanker Limberg , or they could fire a shoulder-fired missile at an airplane taking off or landing, as was attempted in November 2002 against an Israeli charter jet in Mombasa, Kenya. Achieving all four of these objectives is difficult, at best, and in some modes, is practically impossible. Where limited options exist for preventing an attack, policymakers are left with evaluating options for minimizing the consequences from an attack, without imposing unduly burdensome requirements. Following the 9/11 terrorist attacks, Congress took swift action to create the Transportation Security Administration (TSA), federalizing all airline passenger and baggage screening functions and deploying significantly increased numbers of armed air marshals on commercial passenger flights. To this day, the federalization of airport screening remains controversial. For example, Representative Bill Shuster, chairman of the House Transportation and Infrastructure Committee, said in 2015 that, in hindsight, the decision to create TSA as a federal agency functionally responsible for passenger and baggage screening was a "big mistake," and that frontline screening responsibilities should have been left in the hands of private security companies. While airports have the option of opting out of federal screening, alternative private screening under TSA contracts has been limited to 21 airports out of approximately 450 commercial passenger airports where passenger screening is required. Congress has sought to ensure that optional private screening remains available for those airports that want to pursue this option, but proposals seeking more extensive reforms of passenger screening have not been extensively debated. Rather, aviation security legislation has largely focused on specific mandates to comprehensively screen for explosives and carry out background checks and threat assessments. Despite the extensive focus on aviation security for more than a decade, a number of challenges remain, including effectively screening passengers, baggage, and cargo for explosives threats; developing effective risk-based methods for screening passengers and others with access to aircraft and sensitive areas; exploiting available intelligence information and watchlists to identify individuals who pose potential threats to civil aviation; effectively responding to security threats at airports and screening checkpoints; developing effective strategies for addressing aircraft vulnerabilities to shoulder-fired missiles and other standoff weapons; and addressing the potential security implications of unmanned aircraft operations in domestic airspace. Prior to the 9/11 attacks, explosives screening in the aviation domain was limited in scope and focused on selective screening of checked baggage placed on international passenger flights. Immediately following the 9/11 attacks, the Aviation and Transportation Security Act (ATSA; P.L. 107-71 ) mandated 100% screening of all checked baggage placed on domestic passenger flights and on international passenger flights to and from the United States. In addition, the Implementing the 9/11 Commission Recommendations Act of 2007 ( P.L. 110-53 ) mandated the physical screening of all cargo placed on passenger flights. Unlike passenger and checked baggage screening, TSA does not routinely perform physical inspections of air cargo. Rather, TSA satisfies this mandate through the Certified Cargo Screening Program. Under the program, manufacturers, warehouses, distributors, freight forwarders, and shippers carry out screening inspections using TSA-approved technologies and procedures both at airports and at off-airport facilities in concert with certified supply-chain security measures and chain of custody standards. Internationally, TSA works with other governments, international trade organizations, and industry to assure that all U.S.-bound and domestic cargo carried aboard passenger aircraft meets the requirements of the mandate. Additionally, TSA works closely with Customs and Border Protection (CBP) to carry out risk-based targeting of cargo shipments, including use of the CBP Advance Targeting System-Cargo (ATS-C), which assigns risk-based scores to inbound air cargo shipments to identify shipments of elevated risk. Originally designed to combat drug smuggling, ATS-C has evolved over the years, particularly in response to the October 2010 cargo aircraft bomb plot that originated in Yemen, to assess shipments for explosives threats or other terrorism-related activities. Given the focus on the threats to aviation posed by explosives, a significant focus of TSA acquisition efforts has been on explosives screening technologies. However, in 2014, Congress found that TSA had failed to meet key performance requirements set for explosives detection and had not consistently implemented Department of Homeland Security (DHS) policy and best practices for procurement. The Transportation Security Acquisition Reform Act ( P.L. 113-245 ) addressed these concerns by requiring a five-year technology investment plan, and increased accountability for acquisitions through formal justifications and certifications that technology investments are cost-beneficial. The act also required tighter inventory controls and processes to ensure efficient utilization of procured technologies, as well as improvements in setting and attaining goals for small-business contracting opportunities. A major thrust of TSA's acquisition and technology deployment strategy is improving the capability to detect concealed explosives and bomb-making components carried by airline passengers. The October 31, 2015, downing of a Russian passenger airliner departing Sharm el-Sheikh, Egypt, reportedly following the explosion of a bomb aboard the aircraft, has renewed concerns over capabilities to detect explosives in baggage and cargo and monitoring of airport workers with access to aircraft, particularly overseas. In response to a 2009 incident aboard a Northwest Airlines flight, the Obama Administration accelerated deployment of Advanced Imaging Technology (AIT) whole body imaging (WBI) screening devices and other technologies at passenger screening checkpoints. This deployment responded to the 9/11 Commission recommendation to improve the detection of explosives on passengers. In addition to AIT, next generation screening technologies for airport screening checkpoints include advanced technology X-ray systems for screening carry-on baggage, bottled liquids scanners, cast and prosthesis imagers, shoe scanning devices, and portable explosives trace detection equipment. The use of AIT has raised a number of policy questions. Privacy advocates have objected to the intrusiveness of AIT, particularly when used for primary screening. To allay privacy concerns, TSA eliminated the use of human analysis of AIT images and does not store imagery. In place of human image analysts, TSA has deployed automated threat detection capabilities using automated targeting recognition (ATR) software. Another concern raised about AIT centered on the potential medical risks posed by backscatter X-ray systems, but those systems are no longer in use for airport screening, and current millimeter wave systems emit nonionizing millimeter waves not considered harmful. More recently, the effectiveness of AIT and ATR has been brought into question. In 2015, the DHS Office of Inspector General completed covert testing of passenger screening checkpoint technologies and processes to evaluate the effectiveness of AIT and ATR. In testimony, DHS Inspector General John Roth revealed that the covert testing consistently found failures in technology and procedures coupled with human error that allowed prohibited items to pass into secure areas. Even prior to the revelations of weaknesses in passenger checkpoint screening technologies and procedures, the use of AIT was controversial. Past legislative proposals specifically sought to prohibit the use of WBI technology for primary screening (see, for example, H.R. 2200 , 111 th Congress). Primary screening using AIT is now commonplace at larger airports, but checkpoints at many smaller airports have not been furnished with AIT equipment and other advanced checkpoint detection technologies. This raises questions about TSA's long-range plans to expand AIT to ensure more uniform approaches to explosives screening across all categories of airports. Through FY2016, TSA deployed about 750 AIT units, roughly 86% of its projected full operating capability of 870 units. Full operating capability, once achieved, will still leave many smaller airports without this capability. TSA plans to manage this risk to a large extent through risk-based passenger screening measures, primarily through increased use of voluntary passenger background checks under the PreCheck trusted traveler program. However, this program, likewise, has not been rolled out at many smaller airports: currently, the program's incentive of expedited screening is offered at less than half of all commercial passenger airports. In addition to continued deployment and utilization of AIT, the FAA Extension, Safety, and Security Act of 2016 ( P.L. 114-190 ) directed TSA to task the Aviation Security Advisory Committee, composed of industry experts on airport and airline security matters, to develop recommendations for more efficient and effective passenger screening.