The search operation continues deep inside Iran for a second crew member, according to a report by CBS, the BBC's American partner. Combat Search and Rescue (CSAR) missions are considered among the most complex and time-sensitive operations for which the U.S. and allied militaries prepare. In the U.S., elite Air Force units are specially trained for CSAR missions and are often pre-deployed near conflict zones where aircraft might be lost. What is Combat Search and Rescue (CSAR)? Simply put, CSAR missions are military operations aimed at locating, assisting, and potentially rescuing personnel in distress, including downed pilots and isolated troops. Unlike conventional search and rescue, which can occur during humanitarian operations or after disasters, CSAR missions take place in hostile or contested environments. In some cases, as in the reported rescue operation in Iran on Friday, operations can occur deep within enemy territory. CSAR missions are often carried out by helicopters, supported by refueling aircraft and other military planes providing support and patrolling the area. A former commander of a pararescue squadron told CBS News that a rescue operation like the one reported in Iran would involve at least 24 pararescuemen searching the area in Black Hawk helicopters. He added that the team would be prepared to jump from aircraft if necessary, and once on the ground, their priority would be to contact the missing crew member. After locating him, the pararescuemen would provide medical aid if needed, evade the enemy, and reach a location where they could be rescued, according to CBS News. "Dangerous and extremely risky is a mild way of putting it. This is what they train for, all over the world. They are known as the Swiss Army knives of the Air Force," the former commander told CBS News. Verified video that emerged from Iran on Friday shows U.S. military helicopters and at least one refueling aircraft operating over Iran's Khuzestan province. The missions are extremely time-sensitive, as enemy forces would likely be deployed in the same area trying to locate the same U.S. personnel that the CSAR teams are attempting to rescue. Former U.S. Marine Corps special operations expert Jonathan Hackett told the BBC's World Tonight program that the rescue team's priority would be searching for signs of life. "They're trying to work backwards from the last point where they knew the person was, and expand out depending on the speed of movement of that person under different circumstances in this very difficult terrain," Hackett said. Hackett added that this type of rescue would be a "non-standard assisted rescue mission," where local groups in the area might have been pre-contacted with the aim of creating contingency plans that could be activated to assist in the rescue. History of CSAR Missions Aerial combat rescue missions have a long history, dating back to World War I when pilots improvised landings in France to rescue downed comrades. U.S. military rescue units trace their lineage to a 1943 mission when two combat surgeons parachuted into what was then Burma, now Myanmar, to aid wounded soldiers. The world's first helicopter rescue occurred a year later, when a U.S. lieutenant rescued four soldiers behind Japanese lines, according to Smithsonian Air & Space magazine. The incident also marked the first operational use of a helicopter in combat. Formal search and rescue units were first established in the U.S. immediately after the conflict. Modern CSAR began during the Vietnam War. One mission, known as Bat 21, resulted in the loss of several aircraft and multiple American casualties while attempting to rescue a pilot downed behind North Vietnamese lines. The war required a major expansion of CSAR missions with increased scale and complexity. The experience helped the military refine tactics and procedures that laid the foundation for rescues to this day. U.S. Air Force Rescue Teams While each U.S. military branch has its own limited CSAR capabilities, the U.S. Air Force has primary responsibility for locating and rescuing military personnel. This job is primarily carried out by pararescuemen, part of the broader special operations community of the military. The official motto of the rescue unit is: "That Others May Live," and their work is considered part of the broader promise to American soldiers that they will not be left behind. This personnel are highly trained as both combatants and medics, and undergo what is considered one of the toughest selection and training processes in the U.S. military. The selection and training process, which lasts approximately two years from start to finish, includes parachute and dive training, basic underwater demolition training, survival, evasion, resistance, and escape training, as well as a full civilian paramedic course. They also undergo specialized courses in combat medicine, complex rescue operations, and weapons handling. In the field, teams are led by specialized Combat Rescue Officers, who are responsible for planning, coordinating, and executing rescue missions. Recent U.S. Rescue Missions Rescue teams were widely deployed during the wars in Iraq and Afghanistan, carrying out thousands of missions to rescue U.S. and allied soldiers who were wounded or needed evacuation. In 2005, Air Force rescue teams participated in the rescue of a U.S. Navy SEAL who was wounded and sought refuge in an Afghan village after his team was attacked and three other members were killed, an incident later depicted in the film *Lone Survivor*. Rescue missions for downed U.S. pilots have been rare in recent decades. In 1999, an F-117 stealth fighter pilot shot down over Serbia was found and rescued by pararescuemen. In a media-covered incident in Bosnia in 1995, U.S. pilot Scott O'Grady was rescued in a joint Air Force and Marine Corps mission after being shot down and evading capture for six days.