Experts say special counsel Robert Mueller’s use of the Foreign Agents Registration Act against former Trump campaign manager Paul Manafort should worry others reported to have skirted the law's disclosure rules.

FARA is hardly ever used to bring criminal charges, with fewer than 10 cases and one conviction since 1966 for working for foreign political interests without filing paperwork with the Justice Department. Experts see the law's use against Manafort as a sign that may change.

Tougher enforcement may be of particular concern to other subjects of Mueller’s probe into Russia's role in the 2016 election, such as former national security adviser Mike Flynn and Democratic super-lobbyist Tony Podesta, the experts said.

Flynn belatedly registered as a foreign agent of Turkey earlier this year, while Podesta reportedly became a subject of Mueller’s probe over his firm’s belated admission of work with Ukrainian interests. Podesta said Monday he's leaving the Podesta Group amid the probe.

Manafort's receipt of millions of dollars from pro-Russia interests in Ukraine is central to his indictment Friday by a grand jury. The charges were made public Monday, with Manafort and co-defendant Rick Gates alleged to have laundered the money to avoid paying U.S. taxes.

Related: Read the indictment against Paul Manafort and Rick Gates

“In my view it would be unlikely for similarly high-profile matters to be ignored by DOJ for much longer,” said attorney Joshua Rosenstein, a FARA expert at Sandler Reiff Lamb Rosenstein & Birkenstock, P.C.

“I would imagine that this will lead to more aggressive enforcement across-the-board, including against Mr. Flynn and Russian-controlled media outlets, if the facts bear the charges,” Rosenstein said. “I can’t comment on Podesta in particular, but would imagine so.”

Rosenstein said the overlapping alleged crimes committed by Manafort and Gates are generally seen in FARA prosecutions, but that still "the scope of these charges is unprecedented in recent memory: I can’t recall another case where there has been so extensive a reliance on FARA as the basis for an indictment.”

Attorney Ronald Oleynik, a partner and head of the international trade practice at Holland & Knight LLP, said he also could imagine trouble ahead for others, particularly for Flynn.

“Whether that is enough to go after [Flynn] criminally will depend on how much Mueller is out to show that his special investigation was worthwhile,” Oleynik said. “In other words, there is a political element to this. The FARA charges play well because Trump has made foreign interests the bad guy, so it makes Mueller’s job easier to include, and highlight, FARA violations.”

Oleynik said in Flynn's case he "appears to have come clean on the FARA issues fairly quickly, but he does have additional Emoluments Clause problems." (The Emoluments issue relates to his acceptance, as a retired military officer, of $65,000 from Russia-government linked entities for a 2015 trip to Moscow.)

Attorney Daniel Pickard, an expert on FARA and other laws governing corporations with overseas clients at Wiley Rein LLP, agrees that Manafort's indictment may be an indicator of toughening enforcement.

“There is certainly an increased awareness of FARA and there is a perception among many in Washington that we are in a heightened period of enforcement," Pickard said.

In addition to the Mueller probe, the Justice Department currently is reviewing how FARA should apply to Russia-funded but U.S.-based news outlets including RT and Sputnik, alarming some press freedom activists but cheered on by others who see a potential for propaganda.

George Washington University law professor Jonathan Turley said he also finds the use of FARA a potentially worrying sign for others.

“The inclusion of the FARA charges is particularly bad news for potential defendants like Flynn," Turley said.

“These violations are rarely charged, but Mueller is clearly willing to charge for anything short of ripping a label off a mattress,” Turley said. “As anticipated, Mueller is clearly going to charge anyone on anything that he can move before a grand jury.”

Although rare, one recent FARA case ensnared a former high-profile politician, with former U.S. Rep. Mark Siljander, R-Mich., pleading guilty in 2010 for not disclosing that he had been paid by a foreign Islamic charity to convince the Senate Finance Committee to drop it from a list of entities with connections to funding international terrorism.

Craig Holman, a government affairs lobbyist at the advocacy group Public Citizen who has pressed Congress to reform the rarely enforced foreign-agents law to feature civil fines, sees the charges against Manafort and Gates as a possible turning point.

“The entire investigation of Russian involvement in US elections, and possible collusion with Trump operatives, will mean a more aggressive application and enforcement of FARA,” he said. “Manafort’s indictment today, and the likely indictment of Flynn in the near future, will speed up this process.”

Although he supports more enforced disclosure, Holman expressed concern about potential blowback abroad.

“The Department of Justice has historically been a very reluctant ‘cop on the beat’ when it comes to enforcing FARA,” Holman said. “Better enforcement of FARA is long overdue. Unfortunately, it is likely to be used by Putin to further crack down on civil rights NGOs in Russia.”

An attorney for Flynn, Robert Kelner, did not immediately respond to a request for comment, nor did the Podesta Group.