(CNN) Special counsel Robert Mueller has referred a collection of cases to New York federal prosecutors concerning whether several high-profile American lobbyists and operatives failed to register their work as foreign agents, according to people familiar with the matter.

The transfer of the inquiries marks an escalation of Mueller's referrals to the US Attorney's Office for the Southern District of New York in the period since he turned over a case involving President Donald Trump's former personal attorney, Michael Cohen.

Since the spring, Mueller has referred matters to SDNY involving longtime Democratic lobbyist Tony Podesta and his work for his former firm, the Podesta Group, and former Minnesota Republican Rep. Vin Weber and his work for Mercury Public Affairs, the sources said.

One source said that former Obama White House counsel Greg Craig, a former partner at law firm Skadden, Arps, Slate, Meagher & Flom LLP, is also part of the inquiry.

None of the entities involved have been charged with wrongdoing, and there is no indication the SDNY inquiry will result in criminal charges.

It's not clear whether they are considered one case or separate matters, these people said, though all involve inquiries into whether the men improperly performed work on behalf of groups associated with Ukraine without registering with the Justice Department as foreign agents.

Federal law requires that an entity representing a foreign political party or government file public reports detailing the relationship.

At least one of the matters, involving Podesta, was referred to SDNY by Mueller's team shortly before the FBI executed searches of Cohen's home, office and hotel room in early April, according to the sources, though there is no sign that the timing was related to the Cohen investigation. It couldn't be determined if the other matters were referred at the same time.

It's also not clear exactly why Mueller referred the matters to federal prosecutors in New York. Both Mercury and Skadden have offices in Manhattan, though Weber and Craig are based in Washington, DC, and performed the work in question while based in offices there. Podesta and his former firm are also based in Washington. SDNY, however, could claim jurisdiction for reasons such as the location of banking transactions associated with the activity under examination.

Mueller has come under scrutiny from Trump and others for what they perceive as the special counsel's examination of matters beyond the scope of possible collusion between the Trump campaign and Russia during the 2016 election cycle, and Mueller's referral of cases that aren't closely linked to that central matter could be a way of insulating the special counsel probe from such backlash, people familiar with the situation said.

A spokesman for the special counsel's office declined to comment. A spokeswoman for the US attorney's office declined to comment.

A representative for Podesta and the Podesta Group declined to comment.

An attorney for Weber, who led Mercury's Ukraine-linked lobbying work, declined to comment.

"We welcome any inquiry, as we always have, since we acted appropriately at every step of the process, including hiring the best lawyer in Washington we could and following his advice," said Michael McKeon, a partner at Mercury.

Craig declined to comment. Skadden declined to comment.

Mueller had been scrutinizing all three firms as part of his inquiry into former Trump campaign chairman Paul Manafort and his associate, Rick Gates, who were both charged by the special counsel's office with failing to file as foreign agents relating to a decade of work they did for the Party of Regions, a pro-Russia political party in Ukraine.

Manafort, who pleaded not guilty and faces trial on the matter in September, and Gates, who pleaded guilty, paid the Mercury and Podesta firms through a nonprofit, European Centre for a Modern Ukraine, for work that was subsequently revealed to have been done on behalf of Ukraine's government, according to court filings.

Mueller had been examining whether Mercury and the Podesta Group properly identified to federal authorities their foreign advocacy for the European Centre for a Modern Ukraine, CNN previously reported. The ECFMU is a Brussels-based nonprofit group that federal prosecutors have called a mouthpiece for pro-Russian Ukrainian politicians.

The Podesta Group has said it was fully cooperating with the special counsel's office and said it didn't register as a foreign agent for its ECFMU work because that group had misrepresented itself. The Podesta Group said it registered its work for ECFMU instead with Congress, based on what it said was faulty information the non-profit had provided and an outside legal opinion on the matter. The ECFMU had given the firm a statement attesting that it wasn't an arm of a foreign government, the Podesta Group said.

The Podesta Group said previously that it retroactively filed a disclosure after discussions with the Department of Justice.

Skadden performed its own work for Ukraine and also advised Mercury on whether it needed to register its work under the Foreign Agents Registration Act.

Gates admitted as part of his cooperation deal that he had misled Skadden regarding whether the nonprofit was linked to the Ukraine government, a step he knew would avoid triggering Mercury and the Podesta Group to register their work under the FARA statute, according to court filings.

Mercury, too, submitted retroactive FARA filings.

Another former Skadden lawyer, Alex van der Zwaan, pleaded guilty in February to lying to the special counsel's office, admitting that he misled investigators about his discussions related to Skadden's work for the former president of Ukraine, Viktor Yanukovych, in preparing a report on the trial of former Prime Minister Yulia Tymoshenko.

Court filings say that after one of the discussions van der Zwaan lied about, he called the senior partner at Skadden who worked on the report and partially recorded that call. The court papers didn't name the senior partner, but van der Zwaan's attorney identified the partner in a court proceeding as Craig.

The special counsel prosecutor involved in that proceeding, Andrew Weissmann, said in court that van der Zwaan had given an advance copy of the report to a public-relations firm that had been retained by the Ukrainian Ministry of Justice and "had lied to the senior partner on the matter about what he had done."

"When asked about that by the senior partner, he lied and said he had no idea how they got an advance copy," Weissmann said. Weissmann also indicated that Skadden "had insisted on being able to write such an independent report."

Van der Zwaan completed a 30-day prison sentence earlier this year.

Craig exited the Skadden firm in April.