The machines create images outlining the unclothed human body by bouncing X-rays or radio waves off skin or concealed objects. However, security experts say the “advanced imaging technology” has limits: “Backscatter” rays can be obscured by body parts, might not readily detect thin items seen “edge-on” or objects hidden inside the body, and require a human operator to decide whether to conduct additional questioning or a physical search.

“While officials said [the scanners] performed as well as physical pat downs in operational tests, it remains unclear whether the AIT would have detected the weapon used in the December 2009 incident,” the Government Accountability Office, Congress’s audit arm, said in testimony prepared for the hearing…

A TSA spokeswoman said the agency has already conducted a cost analysis and determined that scanners are better than existing alternatives, including metal detectors and machines that check swabs of people’s hands or belongings for traces of explosives. As such, TSA said, the machines increase the odds that security officials can detect anomalies in a fraction of the time and inconvenience as pat-down searches.