Anyone seeking to board a commercial airline flight must be screened by the TSA in order to ensure he is not 'carrying unlawfully a dangerous weapon, explosive, or other destructive substance. Congress has generally has left it to Homeland (D) to prescribe the details of the screening process, which the TSA has documented in a set of Standard Operating Procedures not available to the public. In 2004, Congress gave direct authority to the TSA to 'give a high priority to developing, testing, improving, and deploying' at airport screening checkpoints a new technology 'that detects nonmetallic, chemical, biological, and radiological weapons, and explosives, in all forms.' TSA developed AIT for use at airports. D has procured two different types of AIT scanner, one that uses millimeter wave technology, which relies upon radio frequency energy, and another that uses backscatter technology, which employs low-intensity X-ray beams. Each produces a crude image of an unclothed person, who must stand in the scanner for several seconds while it generates the image. That image enables the operator of the machine to detect a nonmetallic object, such as a liquid or powder - which a magnetometer cannot detect - without touching the passengers coming through the checkpoint. TSA began to deploy AIT scanners to provide additional or 'secondary' screening of selected passengers who had already passed through a magnetometer. TSA decided early in 2010 to use the AIT scanners everywhere for primary screening. Passengers may opt instead for a patdown, which the TSA claims is the only effective alternative method of screening passengers. A patdown may be performed by an officer of the same sex and in private. Many have complained that the resulting patdown was unnecessarily aggressive. With the AIT, each image produced by a scanner passes through a filter to obscure facial features and is viewable on a computer screen only by an officer sitting in a remote and secure room. As soon as the passenger has been cleared, the image is deleted; the officer cannot retain the image on his computer, nor is he permitted to bring a cell phone or camera into the secure room. Also, studies have found that the scanners emit levels of radiation well within acceptable limits. Millimeter wave scanners are also tested to ensure they meet accepted standards for safety. In May 2009 more than 30 organizations, including Ps, sent a letter to D, in which they objected to the use of AIT as a primary means of screening passengers. They asked that the TSA cease using AIT in that capacity pending 'a 90-day formal public rulemaking process.' The TSA ignored their request for rulemaking. Ps made a second request. TSA again responded by letter, clarifying some factual matters, responding to the legal challenges, and taking the position it is not required to initiate a rulemaking each time it changes screening procedures. Ps petitioned this court for review.