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Last month, FBI agents forced a man to unlock his iPhone using the device's built-in facial biometric unlocking system, Face ID.

The FBI forced 28-year-old Grant Michalski's face in front of his iPhone X in order to use his facial biometrics to unlock the device. Investigators then discovered evidence in his chat history—and photos in his possession—of child pornography.

However, Face ID can work against law enforcement because if police officers look to many times at the screen before the subject does, it can "force the iPhone to request a potentially harder to obtain passcode instead," according to Motherboard

In a set of presentation slides obtained by Motherboard, Elcomsoft—a company that specializes in in mobile phone forensics—advised that investigators not look at phones with Face ID, because they might accidentally trigger the requirement for a passcode. Five "wrong" faces will trigger the more stringent security requirement.

Suspects can refuse to reveal passcodes for electronic devices, but courts have forced people to unlock phones with their faces or fingerprints, which don't have the same protections.