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.

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