Special counsel Robert Mueller has issued grand jury subpoenas for public relations firms that worked on an international campaign organized by former Trump campaign manager Paul Manafort, according to NBC News.

The move is the first public indication that Mueller’s team is beginning to compel testimony from witnesses — and suggests that Manafort may be in serious legal peril.

The six firms participated in a public relations effort headed by Manafort, whose ostensible mission was building support for Ukraine’s entry into the European Union, one executive whose firm received a subpoena told NBC News.

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"We think they are trying to figure out, was this a legitimate project?" the executive said. "From our perspective it was — we did a lot of work. We took it seriously.”

Manafort has become a central character in the ongoing investigation into whether President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE’s campaign coordinated with Russia to swing the 2016 election.

He appears to have piqued investigators’ interests with his work for a pro-Russian Ukrainian political party and other international dealings, people familiar with the probe told NBC News. FBI agents raided his Alexandria, Va., apartment last month.

He has also drawn scrutiny for his involvement in the now-infamous 2016 meeting between Donald Trump Jr. and a Russian lawyer who an intermediary claimed had dirt on Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE. Participants in the meeting have said the meeting actually centered on the Magnitsky Act, a slate of sanctions imposed on Russia over human rights abuses that the lawyer was then lobbying to have repealed.

Manafort’s public relations campaign, which ran between 2012 and 2014, was paid for by a Brussels-based nonprofit called the European Center for a Modern Ukraine.

At the time, Ukraine was run by a pro-Russian political party that had paid Manafort millions of dollars in consulting fees in 2013 and 2014. Manafort only belatedly disclosed the $17 million in payments under Justice Department pressure.

The head of the pro-Russian party, former Ukrainian President Viktor Yanukovych, fled to Russia during a so-called “color” uprising in 2014.

Some of the six firms involved in Manafort’s public relations campaign received subpoenas for documents weeks ago, the executive told NBC. The demands for testimony came in recent days.

According to lobbying disclosures, two of the six firms — Podesta Group and Mercury LLC — worked in Washington with Rick Gates, a Manafort partner. Three others worked in Europe, according to the executive. The other three are not known.

The executive said the Brussels nonprofit did not represent itself as funded by a government or political party.

Under federal law, people who lobby on behalf of a foreign government or political party must disclose their activities to the Justice Department — even if they are paid through an intermediary.

Mercury and Podesta Group have since registered under the Foreign Agents Registration Act, at the request of the Justice Department.