Back in 2016, Thomas Fox-Brewster of Forbes documented a case where the US Attorney for the Central District of Florida and the Assistant US States Attorney obtained a warrant requiring everyone at a specific residence to unlock their mobile phones, including those protected by fingerprint sensors.
Forbes included a redacted version of the warrant in PDF format. I have OCRed the Forbes PDF and uploaded it here.
The government submits this supplemental authority in support of its application for a search warrant which seeks authorization to depress the fingerprints and thumbprints of every person who is located at the SUBJECT PREMISES during the execution of the search and who is reasonably believed by law enforcement to be a user of a fingerprint sensor-enabled device that is located at the SUBJECT PREMISES and falls within the scope of the warrant. The government seeks this authority because those fingerprints, when authorized by the user of the device, can unlock the device.
. . .
While the government does not know ahead of time the identity of every digital device or fingerprint (or indeed, every other piece of evidence) that it will find in the search, it has demonstrated probable cause that evidence may exist at the search location, and needs the ability to gain access to those devices and maintain that access to search them. For that reason the warrant authorizes the seizure of “passwords, encryption keys, and other access devices that may be necessary to access the device.”
In Fox-Brewster’s article, Jennifer Lynch of the Electronic Frontier Foundation pushed back on overly broad nature of the warrant,
It’s not enough for a government to just say we have a warrant to search this house and therefore this person should unlock their phone. The government needs to say specifically what information they expect to find on the phone, how that relates to criminal activity and I would argue they need to set up a way to access only the information that is relevant to the investigation.
Fox-Brewster contacted an individual living at the residence who indicated that the warrant had been served, thought no criminal charges had been put forth against any of the individuals at the time the article appeared in October 2016 (the warrant was approved in May 2016).
In iOS 11, Apple introduced a panic mode that, if enabled, will lock the phone after pressing the power button five times quickly. The phone can then only be unlocked by use of the passcode. Android needs to get its act together and implement the same feature as soon as possible.