Special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE has referred several foreign agent inquiries to federal prosecutors in New York in recent months, sources familiar with the matter told CNN on Tuesday.

Based on Mueller's referrals, the U.S. Attorney's Office for the Southern District of New York is investigating whether several high-level American lobbyists violated national laws by failing to register their work as foreign agents on behalf of groups associated with Ukraine, CNN reported.

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Matters referred by Mueller involve Democratic lobbyist Tony Podesta, former Rep. Vin Weber (R-Minn.) and former Obama White House counsel Greg Craig, CNN reported.

Podesta's work with the lobbying firm Podesta Group, Weber's work for Mercury Public Affairs and Craig's work for law firm Skadden, Arps, Slate, Meagher & Flom LLP are reportedly part of the inquiry.

The special counsel's office charged Manafort and Gates with failing to register as foreign agents based on work they did for a pro-Russia political party in Ukraine.

Manfort and Gates paid Mercury Public Affairs and the Podesta Group for work on behalf of Ukraine's government, according to court filings reported by CNN. Manafort pleaded not guilty and faces trial in Washington, D.C., in September, while Gates pleaded guilty.

Mueller reportedly asked prosecutors in Manhattan to look into whether Podesta, Weber and Craig worked on behalf of the Ukrainian government in the U.S. without registering with the Justice Department.

Mueller this year referred President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE's former personal attorney Michael Cohen to the U.S. Attorney's office. Cohen is reportedly under investigation for bank fraud and campaign finance violations.

The referrals have come about as part of Mueller's investigation into ties between the Trump presidential campaign and Russia.

Mueller's probe has come under fire for taking on issues that are extraneous to possible collusion between the Kremlin and the Trump campaign.

Sources told CNN that the special counsel's decision to refer these foreign agent cases to prosecutors in New York should appease those criticisms.