WASHINGTON — There’s more coming.

That's the unmistakable conclusion from documents special counsel Robert Mueller's prosecutors filed in connection with criminal charges against three aides to President Trump's campaign. And the White House and other attorneys involved in the case are bracing for the next shoe to drop.

Administrations officials were surprised by some of the first public charges in the wide-ranging probe into Russia's interference in the 2016 election. Mueller on Monday unsealed indictments of former Trump campaign chairman Paul Manafort and his associate Rick Gates, and revealed that another former aide, George Papadopoulos, had secretly pleaded guilty three months ago to lying to the FBI and had become a “proactive cooperator” with federal authorities.

In public, the White House exuded confidence on Tuesday, insisting the probe will wrap up soon and its policy agenda won't be derailed.

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Yet privately, officials and attorneys who were not authorized to speak publicly about the case speculated about what actions Mueller may take next in an investigation that has clouded the president’s first year in office and left him alternatively frustrated and furious. Some even tossed around possible names who they imagine could be charged next – but acknowledged that the secrecy surrounding Mueller's investigation means they, like everybody else in Washington, are still in the dark. The only thing they seemed to agree on: That Trump himself would not be implicated.

Still, the effect of unsealing the indictments clearly has some of them puzzled. For instance, some officials said they didn't know who Papadopoulos even was until Monday – let alone that he had pleaded guilty to lying to the FBI about his quest to get damaging information on Democrat Hillary Clinton from Russians. Others said they didn't know about the legal exposure of Rick Gates, a Manafort deputy who was indicted on Monday.

So, behind the curtain, officials are wary of a replay of Monday's curveballs – in the form of charges against even more people they didn't know were under investigation.

As Washington lawyer Paul Rosenzweig put it, “Anybody who thinks that this is over is engaged in radically wishful thinking.” Rosenzweig was a prosecutor in an independent counsel investigation of President Bill Clinton.

Investigators are expected to interview White House officials in the coming weeks over Trump's abrupt firing of FBI director James Comey in May and his involvement in the crafting of a controversial statement about his son's Donald Trump Jr.’s meeting with a Russian lawyer in 2016, one official said. The group is likely to include Communications Director Hope Hicks. The official said the White House hopes to have the interviews completed by Thanksgiving.

Yet Trump, in a series of posts on Twitter on Tuesday morning, appeared defensive. He highlighted that the charges against Manafort deal almost entirely with dealings “long before he came to the campaign.” He minimized Papadopoulos's role in the campaign, calling him a young and low-level volunteer – even a "liar." Trump quoted Manafort’s lawyer, Kevin Downing, saying there was “no collusion" between the campaign and Russia – and said that “the Fake News is working overtime” to imply otherwise.

There is, however, one thing the Trump team says is off the table: Any move to dismiss Mueller or end the investigation. Officials say they have been cooperating with the special counsel, and plan to continue even after the recent indictments.

Only the beginning

The case against Papadopoulos, who served as an unpaid foreign policy adviser, brought the first public confirmation by Mueller that a person connected to Trump’s campaign had sought help from people he thought were connected to the Russian government to gather “dirt” — in the form of thousands of emails — on Trump’s rival, Hillary Clinton. Those exchanges came shortly before troves of hacked emails from the Democratic National Committee and Clinton’s campaign manager cast an embarrassing light on her party’s inner workings.

But it also sent another message. Documents Mueller’s office filed in connection with the case make clear that his investigation is far from concluded.

Early this month, when Papadopoulos appeared at a closed-door hearing to plead guilty to a felony count of lying to investigators, a prosecutor working for Mueller, Aaron Zelinsky, told a judge that “there's a large scale ongoing investigation of which this case is a small part.” The transcript of that hearing was unsealed Monday.

Two days before that, FBI agent Jennifer Edwards wrote in a sealed court filing that Mueller’s team was “pursuing leads provided by and related to” Papadopoulos’ email exchanges with other people working on Trump’s campaign.

“The government,” Edwards wrote, “will very shortly seek, among other investigative steps, to interview certain individuals who may have knowledge of contacts between Russian nationals (or Russia-connected foreign nationals) and the campaign.”

The day before that, Mueller’s lawyers secretly persuaded U.S. District Judge Beryl Howell to force one of Manafort’s lawyers to testify before a grand jury investigating him. Mueller, “is investigating foreign interference in the 2016 presidential election and potential collusion in those efforts by American citizens,” Howell wrote in an order unsealed Monday. “This is a matter of national importance.”

A spokesman for Mueller, Peter Carr, declined to comment Tuesday on whether, three weeks later, that remained an accurate assessment of the office’s work.

Still, University of Texas law professor Steven Vladeck said, “the only undeniable takeaway from yesterday is that this is the beginning – and that there’s more to come."

A lot left to learn

Still more telling might be the details prosecutors chose not to include in their filings.

Monday’s legal barrage made no mention of Trump’s decision to fire Comey in May. Mueller’s investigators have asked the White House to turn over documents about Comey’s firing as part of an investigation into whether the president had attempted to obstruct justice by dismissing the man supervising the Russia probe at the time.

The filings also did not mention Michael Flynn, whom Trump fired as his national security advisor after he misled officials, including Vice President Mike Pence, about his contacts with Russian officials after the 2016 election – and who has since come under scrutiny for his work for a firm connected to the Turkish government.

Nor did they mention a meeting Manafort, Donald Trump Jr. and others attended in June 2016 with a Russian lawyer. Mueller’s investigators have sought information that meeting, which Trump's eldest son said he took under the impression he would receive damaging information about Clinton provided by the Russians. The White House later said Trump weighed in on a statement Trump Jr. gave about the meeting, which was revealed to be misleading.

The charges against Manafort and Gates centered on their work on behalf of pro-Russian factions in Ukraine. A grand jury charged that the men failed to register as foreign agents and laundered millions of dollars in profits through overseas shell companies in a bid to avoid U.S. income taxes while using the money to subsidize a “lavish” lifestyle for Manafort that included expensive clothes, rugs and cars.

Rosenzweig and other lawyers said there is no reason to think that the charges Mueller unsealed Monday against Manafort and Gates are an indicator that prosecutors are finished with them.

“In my experience, you pick the pieces you think you can prove against a witness you think you want, and it may not be everything you know, and it certainly isn’t everything you suspect,” he said. There is no requirement that prosecutors bring all of their charges against a person at the same time.

Preet Bharara, the former Manhattan U.S. Attorney fired by Trump earlier this year, speculated that Mueller’s investigative team would try to “flip” Manafort and Gates – seek their cooperation in providing incriminating evidence against “someone who’s higher up in the food chain.” In a special edition of his podcast, Bharara said, “Generally, in the federal system, we try to flip everybody."

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Yet some people caught up in the probe also expressed little fear of Mueller.

Carter Page, who met last week with the Senate intelligence committee also investigating Russia's election meddling, told USA TODAY over the weekend that he had not yet hired a lawyer.

“The past few months has been one of the greatest pleasures of my life,” he wrote in a text message. “You don't fully appreciate the law and a just, functioning legal system, until you've had your basic civil rights so severely abused based on the lies funded by rich political patrons.”

On Monday night, just hours after the charges and guilty plea were unsealed, Page went on MSNBC and said the subject of Russia "may have have come up, yeah," in emails with Papadopoulos.

Heath reported from McLean, Va. Contributing: Steve Reilly in McLean, Va., Kevin Johnson from Washington, and Kevin McCoy from New York.