A federal grand jury has subpoenaed records from a large marketer as part of an investigation into U.S. media buying practices, according to people familiar with the matter, a sign that the probe is advancing even if clients aren't volunteering evidence.

One person familiar with the probe says the subpoena shows several key developments in the investigation, including that a grand jury has been impaneled and that the U.S. attorney has accumulated enough evidence to convince the jury of probable cause to subpoena a client's records.

The subpoena comes after the Association of National Advertisers last fall announced that the Federal Bureau of Investigation had approached the group seeking cooperation. The ANA issued a white paper on how marketers could cooperate, but there's no evidence any marketers have voluntarily come forward.

The subpoena, however, suggests that's not stopping the FBI or Justice Department from going after client records anyway. People familiar with the matter say the client who received the subpoena came to the ANA for guidance on how to deal with it. One of these people described the client as having a mid-nine-figure media budget and that the subpoena seeks two years of financial records, e-mails and other communications between the client and its agency. These people declined to name the client and the agency, citing confidentiality requirements.

While the federal probe puts further pressure on agencies, the client subpoena shows it could also lead to difficulties for marketers. Complying with the subpoena will require legal fees and substantial record collection and analysis, and it could take years for any charges to be filed or restitution to be ordered, if ever.

FBI has names in K2 report

One of the people familiar with the probe says the FBI also has an "unredacted" version of the report K2 Investigations completed in 2016 for the ANA on rebates and other "non-transparent" media practices. The unredacted report includes names of all 41 unnamed sources.

The Justice Department didn't respond to a request for comment by deadline.

ANA CEO Bob Liodice said in an e-mail that he knows nothing of the client subpoena. He added that under terms of the group's agreement with K2, the association itself never got to see names of unidentified sources in the report.

"We have been more than at arms-length in those situations," Liodice says. "Unless there is someone on my staff that accidentally bumped into more detailed and explicit information, I am not aware of anyone at ANA with any general or specific knowledge of anything."

Rich Plansky, former executive VP of K2 who led the investigation behind the ANA report, said at the time it was released that his firm had no lawyer-client or other privilege to shield unnamed sources from investigators if the firm were asked or subpoenaed. Plansky, now with another investigations firm, Exiger, reiterated that this week but otherwise referred questions to K2.

K2 is 'fully cooperating'

"K2 Intelligence is fully cooperating with law enforcement looking into this matter," K2 Global Chief Marketing Officer Paula Zirinsky said in an e-mail statement, but declined to comment further.

One person, who said he was an unnamed source for K2, said he hasn't personally been contacted by the FBI but knows of two others who have been. Another person who was a source for the report says the FBI began asking questions last April, mainly seeking clarifications about industry terminology and practices. This person said advice from his own counsel suggests K2 may have been legally obligated to volunteer any evidence of potential criminal violations to the Justice Department. Last April also was when the FBI first approached the ANA seeking cooperation, another person familiar with the matter says.

Evidence sought by the FBI could span many years, because any statute of limitations on fraud or other original infractions may not be a constraint. In its November guidance for members, the ANA's law firm, Reed Smith, noted that wire and mail fraud may ultimately become the focus of the probe. The statute of limitations on those offenses is five years in most cases and as much as 10 years if a financial institution is involved, and the clock only starts ticking after the last infraction occurs.

That might include any time an agency used e-mail, phones or the mail to reassure clients that either the agency never got rebates on their business or passed all rebates along to clients--if evidence ultimately proves those reassurances false. The search for evidence of rebates or other non-transparent transactions could then stretch back many years prior.

The ANA pointed out that any assurances agencies made after former Mediacom CEO and ANA consultant Jon Mandel gave his blistering speech in 2015 at the group's Media Conference about widespread unreported rebates in the U.S., or after the K2 report was issued in 2016, could be subject to wire-fraud charges if those assurances were untrue.

Mandel's speech set off intense scrutiny of U.S. media agency practicies, prompting clients to re-draw contracts with stricter rules and tougher audit provisions. Practices under the microscope include undisclosed rebates, or transactions where agencies operated as principals in media deals without disclosing their role or getting authorization from clients. But some people familiar with industry practices say problems persist today.

Some recent audits conducted by marketers have led to an eight-figure settlement in Europe and a seven figure settlement in the U.S., according to people familiar with the matter. These people declined to discuss details, citing non-disclosure agreements that are included as part of the settlements.

Some ANA members fear 'retaliation'

The Reed Smith report for ANA members in November noted that some marketers have reported fear of being "blacklisted" by agencies or other retaliation if they're found to be cooperating with investigators. The report said federal investigators, however, go to great lengths to shield identities of cooperating witnesses as long as possible. That may not be possible if cases advance to pre-trial discovery, but the cases are more likely to be plea bargained before that, the report says. Even if they're not, agencies or others found to be retaliating against witnesses or cooperating companies would be subject to additional criminal charges with stiffer penalties than fraud charges are likely to have.

The 4A's, the primary trade group representing U.S. ad agencies, declined to comment. "As a practice, we don't comment on speculations surrounding ongoing investigations," a spokeswoman said.

A spokeswoman for Omnicom Media Group said the holding company has not been subpoenaed in the investigation but declined to otherwise comment.

Interpublic in an e-mail statement said: "As a leader in media transparency, we continue to be confident of our position as a trusted partner to our clients." IPG Chairman-CEO Michael Roth has been on the record about having talked voluntarily to K2 for its report in 2016.

Spokespeople for WPP, Publicis Groupe, Dentsu Aegis Group, Havas and MDC Partners didn't comment or respond to requests for comment by deadline.