In an open letter to members of the disability community, a top Transportation Security Administration official says that some people with disabilities will be required to undergo “alternate screening techniques including pat-downs.”

The letter sent Monday was designed to clarify airport screening techniques ahead of the Thanksgiving travel rush. It comes as more airports shift their screening procedures from metal detectors to full-body scanners. Those who opt out of the machines or who trigger alarms are subject to intense pat-downs, which have been sharply criticized by some passengers who claim that they are too invasive.

However, TSA officials say some people with disabilities are ineligible for the body scanners and therefore must automatically undergo secondary screening measures. This includes individuals who use wheelchairs or other mobility devices and cannot stand, travelers with service animals, people who rely on a cane or walker and those unable to lift their arms to shoulder level for several seconds.

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Similarly, travelers who are accompanying or assisting a person who cannot pass through the machines will also be subject to alternative screening measures, wrote Kimberly Walton, the TSA’s special counselor in the letter.

“There is nothing punitive about our measures; it just makes good security sense,” Walton wrote. Nonetheless, she acknowledged, “the pat-down you receive will be more thorough than what you may have received previously.”