Uber has had this habit of deploying its app and drivers cities where, strictly speaking, using Uber may not yet be legal. You’d think it would be easy for law enforcement in those cities to nab rogue Ubers: Just use the app to hail a car and arrest or cite whoever shows up. However, it looks like Uber figured out how to sidestep these snares.

The New York Times reports that the service developed and used a tool called Greyball that specifically shunted users suspected of being law enforcement into an alternate, non-functional version of the Uber app.

An inspector in Portland, OR, managed to capture Greyball in action on video in 2014, the Times reports. He was trying to hail an Uber car as part of a sting against the company, which was operating without permission in Portland. (The city later declared the service illegal.)

He requested a car, just like any other user — but the digital cars he saw on his screen were deliberately fake “ghost” cars that could not give him rides.

Greyball is part of a bigger program Uber calls VTOS — for “violation of terms of service.” In a statement to the Times, Uber said, “This program denies ride requests to users who are violating our terms of service — whether that’s people aiming to physically harm drivers, competitors looking to disrupt our operations, or opponents who collude with officials on secret ‘stings’ meant to entrap drivers.”

The company’s legal team signed off on VTOS and Greyball, the Times reports, despite the fact that it may move into murky ethical and legal territory.

Uber “greyballs” users “based on data collected from the app and in other ways,” the Times reports. That included drawing a virtual perimeter around regulators’ office buildings, so that Uber could monitor who frequently opened and then closed the app from inside that boundary.

Uber also checked against users’ payment information. If the credit card on file is tied directly to, say, a police credit union, well, you’ve got pretty good odds that the user is a cop.

Nor did using burner phones help a city employee avoid being greyballed, the Times adds: Uber employees went to local electronics stores in some cities, noted the device numbers of the cheapest mobile phones on sale, and then compared those to the phones being used by suspected city officials.

“In all,” the Times writes, “there were at least a dozen or so signifiers in the VTOS program that Uber employees could use to assess whether users were new riders or very likely city officials.” And that doesn’t include simply Googling someone and looking at their available social media profiles, which Uber also did.

It’s not all necessarily nefarious, the Times adds; in certain other nations, greyballing users could protect Uber drivers, who have been the target of violence from taxi workers in those regions. It also uses greyballing to protect itself from collusion between the local transportation authority and taxi companies in some cities in the U.S., Uber told the Times.

Legal experts who don’t work for Uber had more doubts about the legality of the tool than Uber’s in-house counsel, the NYT notes.

“With any type of systematic thwarting of the law, you’re flirting with disaster,” one told the Times. “We all take our foot off the gas when we see the police car at the intersection up ahead, and there’s nothing wrong with that. But this goes far beyond avoiding a speed trap.”

How Uber Used Secret Greyball Tool to Deceive Authorities Worldwide [The New York Times]

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