In a signal that the FBI’s investigation into media buying and transparency is progressing, the FBI has reportedly interviewed former and current executives from multiple media buying agencies, and reports surfaced in March that the FBI had issued a subpoena seeking information from an agency client.

A person familiar with the matter told Ad Age that the subpoena signaled several key developments in the investigation, including that a grand jury has been impaneled and that the U.S. attorney has accrued enough evidence to convince the jury of probable cause to subpoena a client’s records. Though there’s no evidence that any marketers have voluntarily come forward to aid the FBI in its investigation, the agency appears poised to go after client records regardless. The FBI is reportedly using an unredacted version of the K2 report on media rebates, which includes the name of all 41 sources previously redacted. Part of the FBI’s investigation has reportedly zeroed in on the out-of-home advertising sector, which generated $10 billion in ad revenues in 2018, according to the Interactive Advertising Bureau.