Looking into Uber’s attempt to evade government regulators in Portland, Oregon, the U.S. Department of Justice has launched a criminal probe into the development and use of Uber’s Greyball software. Detailed by Reuters, a Northern California grand jury sent a subpoena to Uber recently specifically requesting information on where Greyball was deployed and how the software functions.

In cities where ridesharing services like Lyft and Uber are illegal, government officials will occasionally launch sting operations to bust drivers. Basically, a police officer will hail an Uber car in order to ticket the driver and potentially impound the vehicle. This dissuades drivers from operating Uber vehicles within the city and increases the number of residents using licensed taxi services.

Enter Grayball, a piece of software that analyzes information like credit card data or social media profiles to filter out unwanted users. For instance, the software may identify a police officer who operates these types of stings and hide all nearby drivers within the app. Basically, the software either showed fake drivers on the app screen or simply showed zero cars available in the area. Those fake cars would never respond to the officer’s requests.

While Uber claims that the software is designed to identify and filter out taxi union protesters that could potentially harm its drivers, the company announced that Greyball would no longer be used for filtering out government officials after the New York Times exposed Greyball’s existence during March 2017.

According to sources knowledgeable of the probe, the federal investigation is currently in the early stages. At this time, neither the U.S. Department of Justice nor Uber has released an official comment on the status of the investigation. However, Uber has retained the services of a third-party law firm to investigate usage of Greyball software within the company.