Robert Mueller. AP Photo/Andrew Harnik

Special counsel Robert Mueller's team has subpoenaed a number of Washington lobbying firms as part of its investigation into whether President Donald Trump's campaign colluded with Moscow to tilt the 2016 election in his favor, The Washington Post reported on Friday.

The firms that were subpoenaed have been associated with consulting firms led by former national security adviser Michael Flynn and former Trump campaign manager Paul Manafort. Both Flynn and Manafort are being scrutinized as part of the Russia investigation.

Manafort left the Trump campaign last August, and Flynn was forced to resign as national security adviser in February, when it emerged that he had misled Vice President Mike Pence about his contacts with Sergey Kislyak, the former Russian ambassador to the US.

The subpoenas come as both Flynn and Manafort face questions about allegedly failing to disclose the lobbying work they did on behalf of foreign governments, in violation of US federal law.

The Post reported that two of the subpoenas were issued to SGR LLC and Mercury Public Affairs, citing sources familiar with the matter. A lawyer representing SGR LLC confirmed to The Post that the firm had received a subpoena, and people close to Mercury Public Affairs said they had also received requests, though the company declined to comment.

Ukraine lobbying work

Mueller's team reportedly asked Mercury about public relations work it had conducted for the European Centre for a Modern Ukraine at Manafort's request. The organization's stated goal is to foster closer ties between Ukraine and the European Union, as well as the United States. It was founded, however, by Leonid Kozhara, a senior member of parliament for Ukraine's pro-Russia Party of Regions.

Manafort is linked to the party through his time serving as a top adviser to Ukrainian President Viktor Yanukovych, a pro-Russian strongman whom Manafort is widely credited with helping win the presidency in 2010.

Yanukovych was ousted in 2014 after widespread demonstrations again this decision to back out of a deal with the EU that would have distanced Ukraine from Russia and strengthened ties with the West. Yanukovych fled to Russia amid the protests, during which Ukrainian riot police opened fire on thousands of demonstrators, and is now living under the protection of the Kremlin.

In this July 18, 2016, file photo, Trump campaign chairman Paul Manafort walks around the convention floor before the opening session of the Republican National Convention in Cleveland. AP Photo/Carolyn Kaster, File

Ukrainian prosecutors have said Yanukovych ordered the security forces' attack on protesters, and at least one human-rights lawyer representing the victims is investigating what role, if any, Manafort played in encouraging Yanukovych's crackdown.

Mercury worked with Podesta group — the lobbying firm led by John Podesta's brother, Anthony Podesta — on the Ukraine lobbying project, and the firms did not register as foreign agents at the time, saying they were working for a nonprofit and not a foreign government or political party, The Post reported. Both firms recently registered retroactively, however, acknowledging that the Party of Regions benefited from their work.

Manafort has drawn increased scrutiny from the FBI in recent months. The Washington Post reported that the bureau conducted a predawn raid on his home in July, and agents working with Mueller left Manafort's home "with various records."

Manafort has been cooperating with investigators' requests for relevant documents. But the search warrant obtained by the FBI in July indicates that Mueller managed to convince a federal judge that Manafort would try to conceal or destroy documents subpoenaed by a grand jury.

Turkey lobbying work

Mueller's team subpoenaed SGR after Flynn's lobbying firm, Flynn Intel Group, hired the group to ostensibly "promote a good business climate in Turkey," The Post reported. Flynn's firm hired SGR as part of its work with a Dutch company with links to Turkish President Recep Tayyip Erdogan.

Although Flynn's group's initial stated goal in hiring SGR was to foster a stronger business climate in Turkey, it was later forced to indicate that it brought SGR on to "raise concerns" to the US about Fethullah Gulen, a Turkish cleric currently living in exile in Pennsylvania whom Erdogan has blamed for mounting a failed coup against his government and fomenting dissent within Turkey.

Michael Flynn. AP Photo/Carolyn Kaster

Flynn's lobbying group has undertaken other efforts to discredit Gulen and bolster Erdogan as well. The firm reportedly did not want anyone to know that it was working on a documentary commissioned by a Turkish businessman to help Turkey's image in the wake of last July's failed coup.

The businessman, Ekim Alptekin, paid the Flynn Intel Group over $500,000 to produce a documentary about the dangers of Gulen, he told The Wall Street Journal in May. "We thought that might have a good effect," he added. The film was never finished.

Flynn additionally raised eyebrows when he wrote an op-ed for The Hill, published on November 8, alleging Gulen helmed a "vast global network" that had "all the right markings to fit the description of a dangerous sleeper terror network."

In addition to being scrutinized as part of Mueller's probe into whether the Trump campaign colluded with Russia in 2016, Flynn is also under a separate FBI investigation for privately working as a paid lobbyist for Turkey during the 2016 campaign. Flynn informed the Trump transition team of the investigation in early January, weeks before Trump was inaugurated.

He did not resign until mid-February, after it was reported that he had discussed US sanctions against Russia with Kislyak during the transition period and misled Pence about the conversations.

Flynn has asked for immunity from prosecution from the bureau and the House and Senate intelligence committees in exchange for his testimony. It is unclear whether the FBI has granted Flynn immunity, but the intelligence committees have ruled it out.

Natasha Bertrand contributed reporting.