According to a report today from CNN, Google is facing a fine which could be in excess of $500m after a federal investigation into pharmaceutical ads carried by the search firm.

Prosecutors in Rhode Island and undercover agents from the Food and Drug Administration are said to be attempting to prove that Google knowingly carried ads for websites selling highly addictive drugs without a legitimate prescription.

CNN goes on to report that in May, Google notified the Securities and Exchange Commission that it was setting aside $500m in connection to a case with the Department of Justice related to “The use of Google advertising by certain advertisers.”

Google’s self-service AdWords platform leaves it open to potentially carrying advertising for illegal products and services, and legal experts reportedly told CNN that a $500m+ fine would be “the largest such penalty in U.S. history.” However, both the U.S. Attorney’s office in Providence, Rhode Island and Google declined to confirm the existence of the Google declined comment or confirm the investigation’s existence.