Special counsel Robert S. Mueller III testifying before the Senate Judiciary Committee. (Saul Loeb/Agence France-Presse/Getty Images)

A little-noticed court filing unsealed this week as part of special counsel Robert S. Mueller III’s ongoing probe could have big consequences for his other targets — showing he’s willing to use suspects’ lawyers to provide evidence against them.

After unsealing a 12-count indictment against President Trump’s former campaign chairman, Paul Manafort, the U.S. District Court also unsealed an opinion from Chief Judge Beryl A. Howell saying one of Manafort’s former lawyers could be compelled to testify to the grand jury.

Typically, such information is protected by attorney-client privilege — a bedrock principle of U.S. legal practice that says a lawyer must keep confidential what they are told by their clients.

There are some exceptions to that confidentiality, including in instances where a suspect may have lied to his or her lawyer, causing that lawyer to unwittingly lie to the government. Howell ruled that in Manafort’s case, the exception applied and the attorney could be called to testify before the grand jury.

“The opinion is troubling, because people make representations to the government all the time through their lawyers, and I think there’s a general expectation of confidentiality behind the conversations that go into those representations,’’ said Peter D. Hardy, a partner at the Ballard Spahr law firm. “It’s widening a door that’s not often used. And the wider the door gets, maybe the more people will use it.’’

The Post's Matt Zapotosky and Devlin Barrett explain what could come next following the indictment of former Trump campaign chairman Paul Manafort and the guilty plea of former campaign adviser George Papadopoulos. (Joyce Lee,Bastien Inzaurralde/The Washington Post)

Hardy said the use of what lawyers call the “crime-fraud exception’’ indicates “the special counsel team is highly intelligent, highly aggressive, and they’re going to pursue legal theories that your average prosecutor will not use. Will they use the specific theory again? It’s certainly possible.’’

Manafort is hardly the only person under scrutiny who used lawyers to make government filings. One of Mueller’s other key targets, Michael Flynn, has also filed government papers regarding his work on behalf of another government.

No public charges have been filed against Flynn, former national security adviser in the Trump administration, but he remains under investigation, according to officials familiar with the matter who, like others, spoke on the condition of anonymity to discuss an ongoing investigation.

In the Manafort case, the judge began her ruling by declaring, “This is a matter of national importance.’’ The ruling does not identify Manafort’s lawyer — it refers to the attorney simply as “the Witness,’’ but people familiar with the case said it was Melissa Laurenza who was willing to testify if her clients consented, which they did not. Neither Laurenza nor her lawyer responded to a request for comment Tuesday.

A Manafort spokesman also declined to comment.

After several hearings on the subject, the judge decided prosecutors could ask the lawyer seven out of eight proposed questions, ruling one went too far. The judge said investigators had gathered enough evidence from other sources to justify questioning the lawyer, though the details of that evidence were redacted from the court filing.

Here’s what we know about the charges and a timeline of events.

That legal fight took place under seal in September, but the opinion was unsealed Monday, the same day charges were announced against Manafort and his deputy Rick Gates.

Prosecutors also revealed Monday that a former Trump campaign adviser, George Papadopoulos, had pleaded guilty and been cooperating with investigators for months.

In Manafort’s case, prosecutors used information from his former lawyer to charge him with lying to the government about his work for a foreign government — Ukraine. While most of the indictment focuses on issues of alleged money laundering, conspiracy and failure to file reports to tax authorities, the last two counts of the indictment say that Manafort and Gates “knowingly and willfully caused to be made a false statement’’ in a government filing required under the Foreign Agents Registration Act, or FARA.

Court filings suggest those two charges stem in part from information provided by Manafort’s former lawyer.

In the FARA filing, Manafort and Gates denied that their work on the part of Ukraine’s Party of Regions constituted lobbying on behalf of a foreign government.

Jacob Frenkel, a white-collar attorney at Dickinson Wright who previously worked in the Office of the Independent Counsel, said Howell’s ruling was likely a critical one for prosecutors — who otherwise “would need to jump over and avoid altogether the communications with counsel.”

“It is invaluable to the FARA counts, and to the issue of Manafort’s state of mind more broadly in the indictment, because an experienced, credible lawyer is going to have instant credibility with a jury,” Frenkel said.

Frenkel said that the ruling was written in such a way that Howell seemed to believe it would be appealed.

“The attorneys’ testimony will be central to an actual case, if this case ever gets there,” Frenkel said.