With the guilty plea of one Trump campaign official and a 31-page indictment of two others, special counsel Robert S. Mueller III spoke volumes more about the Russia probe than months of heated public debate. Without uttering a word, Mueller's message was clear, according to veteran lawyers: He isn't bluffing, and witnesses are talking.

The double-barreled court filings ratchet up the pressure on everyone under scrutiny in the investigation, lawyers said, in part because they show that a former Trump campaign adviser began cooperating with the FBI three months ago.

"This is the way you kick off a big case,'' said Patrick Cotter, a white-collar defense lawyer in Chicago who once worked as a federal prosecutor in New York alongside Andrew Weissmann, who is spearheading the prosecution of former Trump campaign chairman Paul Manafort and his deputy, Rick Gates.

Prosecutors also announced a guilty plea from former campaign adviser George Papadopoulos about his interactions with people linked to the Russian government. Papadopoulos has admitted to lying to FBI agents who questioned him about those contacts, according to court ­records.

"Oh man, they couldn't have sent a message any clearer if they'd rented a revolving neon sign in Times Square,'' Cotter said. "And the message isn't just about Manafort. It's a message to the next five guys they talk to. And the message is: 'We are coming, and we are not playing, and we are not bluffing.' ''

The special counsel also appears to be moving fast.

White-collar cases often begin by "flipping'' bit players far down the chain of command. The ­prosecution team that sent WorldCom chief executive Bernie Ebbers to prison for decades began with charges against a low-level accountant. Prosecutors then worked their way up the corporate ladder, flipping senior managers until they had a case against Ebbers.

And as Mueller probes whether Trump associates conspired with Russian government actors to influence the 2016 presidential election, he already has a cooperator at the lower rungs of the campaign: Papadopoulos. The prosecutors' court filings say that Papadopoulos sought to arrange a meeting between officials with the Trump campaign and the Russian government, and talked to people connected to the Russian government about providing "dirt'' on Democratic candidate Hillary Clinton.

Peter Zeidenberg, a former federal prosecutor with expertise in national security, called the Papadopoulos plea "a big deal'' because it "goes much closer to the issue of collusion.''

The fact that he's been cooperating for three months is important, too, he said. "Who else is cooperating that we don't know about? That's what people in the White House need to be worried about.''

While the Papadopoulos court filings offer more links between Trump associates and Russian figures, they do not define the extent to which senior campaign officials supported that effort.

"It's not a direct connection as yet," said Daniel E. Toomey, a former prosecutor in Washington. "There are additional bread crumbs that are missing leading to the White House.''

Toomey called the indictment of Manafort and Gates "extraordinarily intricate'' and said it will put "enormous pressure'' on the defendants to cooperate.

[The 12-count Manafort and Gates indictment, annotated]

"This becomes a paper case. The documents speak for themselves,'' he said. "This case, by my estimation, will take months to try. The cost of the litigation for Manafort will be in the millions of dollars."

Veteran lawyers described the indictment as daunting — alleging $18 million worth of money laundering, which prosecutors said in court could mean more than a dozen years in prison.

The indictment even seems to go out of its way to kneecap a standard defense argument in such cases — that the defendant got bad advice from his accountant. The indictment charges that Manafort wrote specifically to his accountant that he did not have foreign bank accounts.

[Read the George Papadopoulos charges and plea deal]

Nick Akerman, a former assistant special Watergate prosecutor, said the court filings "all spell bad news for President Trump.''

Akerman said he could not see any defense to the Manafort indictment.

"He has no choice but to plead guilty. That's what the indictment says to me,'' he said. "The only defense that you've got is to go in there and start singing like a canary to avoid jail time. And once he starts singing, one of the tunes is bound to be Donald Trump.''

But Jay Nanavati, a former Justice Department tax prosecutor, said the filing of the indictment shows that so far, prosecutors have "not been able to convince Manafort to cooperate, but this is still how you start moving up the ladder in any organization.''

What's striking about the indictment, Nanavati said, is the number of people who worked for Manafort — accountants, lawyers and others — who provided key evidence against him.

That could be an ominous warning for other subjects of the probe, like former national security adviser Michael Flynn, who used lawyers to make official statements to the government that have now come into question.

And unidentified lobbying firms indirectly hired by Manafort, he said, "are in significant trouble'' because of the written exchanges referenced in the indictment alleging that some people at the firms were aware that Manafort was lying about their work together.

Nanavati said a key change in U.S. tax enforcement that began in 2008 — going after foreign bank accounts controlled by Americans — probably played an important role in the case against Manafort. Before the Justice Department started cracking through the layers of Swiss banking secrecy that hid Americans' accounts in 2008, he said, the requirement on Americans to declare foreign bank accounts was on the books but "basically, nobody ever did it, and almost no one ever got prosecuted for it. That changed in 2008, and tax return preparers started asking.''

With the decline of bank secrecy, Nanavati said, crimes such as tax evasion and money laundering through foreign accounts became harder to hide, and made the charges against Manafort easier to bring.

A key leader of the tax enforcement effort was then-Justice Department lawyer Kevin Downing.

Downing is now Manafort's defense lawyer. Outside court on Monday, he blasted the government's indictment for claiming his client committed crimes via foreign bank accounts. To claim that maintaining such accounts, "to bring all your funds into the United States, as a scheme to conceal from the United States government, is ridiculous,'' Downing said.

Read more:

Paul Manafort: A FAQ about Trump’s indicted former campaign chairman

How George Papadopoulos tried to work with the Russian government