Uber is facing the possibility of criminal charges from the Justice Department over software that allows drivers to evade local traffic regulators, according to a report Thursday evening.

Though the existence of a federal probe has already been reported, Reuters cited two sources familiar with the situation who say a criminal investigation is underway.

At issue is the software tool, "Greyball," which allowed the company to keep tabs on and steer drivers clear of government officials tasked with enforcement in areas where Uber's ride-share service has not yet been approved.

The software tool, via Uber's app, might also have been used to mine credit card information to identify any affiliation with a credit union used by police and monitored users' social media accounts for ties to law enforcement, the report said. Current and former employees said it worked as part of a wider program, called, Violation of Terms of Service, which analyzed personal and financial information to determine if a ride request was legitimate.

Uber's lawyers have already admitted that Greyball has been used. In a report already made public by Portland, Ore., Uber lawyers told city authorities that the software tool was used "exceedingly sparingly" there.

A Northern California grand jury has subpoenaed the company in an effort to obtain document about how Greyball was used, one source told Reuters.

The criminal violations the Justice Department may be looking for are not immediately clear.