The U.S. Department of Justice has begun a criminal investigation into Uber Technologies' use of a software tool that helped its drivers evade local transportation regulators, two sources familiar with the situation said.

Uber has acknowledged the software, known as " Greyball ," helped it identify and circumvent government officials who were trying to clamp down on Uber in areas where its service had not yet been approved, such as Portland, Oregon.

The company prohibited the use of Greyball for this purpose shortly after the New York Times revealed its existence in March, saying the program was created to check ride requests to prevent fraud and safeguard drivers. The Times report triggered a barrage of negative publicity for the company.

The criminal probe could become a significant problem facing the company that is already struggling with an array of recent business and legal issues.

An Uber spokesman and the Justice Department declined to comment. Uber lawyers said in letters to Portland authorities, which Portland made public in a report last week, that the Greyball technology was used "exceedingly sparingly" in that city, before the service was approved there in 2015.

The nature of any potential federal criminal violation, and the likelihood of anyone being charged, is unclear. The investigation is still in its early stages, the sources said.