First came George Papadopoulos, the former Trump campaign adviser who was arrested by the FBI when he stepped off a plane at Dulles International Airport and soon agreed to help the special counsel’s office as part of a plea agreement.

Then there was Michael Flynn, the president’s former national security adviser who admitted he lied to the bureau and would now be cooperating with Robert S. Mueller III’s team to make things right.

Next to fall was Rick Gates, Trump’s former deputy campaign chairman who conceded he conspired to defraud the United States and tried to deceive investigators looking into his overseas work.

One by one, the special counsel’s office methodically turned allies of President Trump into witnesses for its investigation — irking the commander in chief so much that he has suggested the commonplace law enforcement tactic “almost ought to be illegal.” But former Trump campaign chairman Paul Manafort had long eluded Mueller’s team, with his resistance to a plea deal so intense that some in law enforcement figured he must know he would soon receive a pardon.

Before he joined the Trump campaign, Paul Manafort made a name for himself in the D.C. lobbying world, but his past caught up with him. (Dalton Bennett, Jon Gerberg, Jesse Mesner-Hage/The Washington Post)

On Friday, though, the special counsel finally nabbed his white whale. Manafort, whose role in the Trump campaign and ties to a Russia-aligned strongman and a suspected Russian intelligence agent make him an enticing cooperator, pleaded guilty to conspiracy to defraud the United States and obstruction of justice. As part of his agreement with prosecutors, he said he would tell the special counsel’s office all that he knows.

Manafort’s plea could be a key cog in pushing Mueller’s case toward its ultimate end. Legal analysts say Manafort must have something valuable to share with Mueller’s team, which agreed to drop five of the seven charges he faced and potentially urge leniency at his sentencing, if his cooperation is helpful.

Generally, those who plead guilty sit down with prosecutors to detail what they know in a “proffer” session, so the government knows what it will get in the bargain. Manafort’s plea makes reference to a written proffer agreement on Tuesday — showing he has been in talks with the special counsel’s office at least for several days.

Whether Manafort ultimately implicates the president remains to be seen. Manafort’s defenders and Trump’s lawyers have long insisted that the political consultant, who left the campaign in August 2016, had no information that would incriminate Trump.

“I think Robert Mueller’s real quest here is for the truth, and Paul Manafort can get him closer to knowing the truth,” former U.S. attorney Barbara McQuade said.

Trump attorney Rudolph W. Giuliani said Friday that it would be impossible for Manafort’s cooperation with Mueller’s office to imperil the president. That is because Trump and Manafort continued to have a joint defense agreement — an informal arrangement among lawyers to share information — which Manafort would have to cancel if he believed his cooperation could expose Trump to legal jeopardy, Giuliani said.

Inside the White House on Friday after the plea, the mood was “oddly calm,” said one Republican in frequent touch with officials there. A number of people had expected some sort of agreement, and Trump’s legal team recognized it couldn’t control Manafort’s desire to avoid a second trial after being convicted on eight of 18 counts in a related case in Virginia last month.

Trump himself has not yet addressed the plea directly.

The charges to which Manafort pleaded guilty had nothing to do with the president. Rather, they focused on Manafort’s personal money laundering, failure to register as a foreign agent for work he did on behalf of Viktor Yanukovych, the pro-Russian former president of Ukraine, and obstructing justice with Konstantin Kilimnik, whom prosecutors have linked to Russian intelligence.

White House press secretary Sarah Huckabee Sanders noted that point in a statement responding to the development.

“This had absolutely nothing to do with the President or his victorious 2016 Presidential campaign,” she said.

But while the White House projected confidence about its position, some officials privately acknowledged that they could not be sure what Manafort might expose about the campaign or about interactions with Russians.

Manafort was a participant in the now-infamous June 2016 Trump Tower meeting, where the president’s son Donald Trump Jr. and son-in-law, Jared Kushner, sat down with a Russian lawyer thinking they would get damaging information on Hillary Clinton. He also was a part of the Trump campaign when the Republican Party platform was changed in a way viewed as more favorable to Russia because it did not include support for arming Ukraine.

“I think he potentially knows a lot of information, just in light of his role as the campaign chairman during that crucial time during the summer of 2016,” said McQuade, who watched much of Manafort’s first trial.

Manafort’s plea agreement short-circuited a trial in the District that was scheduled to begin in coming days with jury selection. He instead agreed to admit wrongdoing and cooperate fully with Mueller, turning over any documents that may be relevant to the special counsel’s investigation and testifying in any proceedings where that might be necessary. He also agreed to give up five properties and a handful of financial accounts.

Having already been convicted in Virginia, Manafort’s cooperation might be the best way for him to reduce his time in prison. He faces roughly 10 years in the D.C. case and perhaps another 10 in Virginia — though he would probably be able to serve those together, particularly if prosecutors urge judges to go easy on him.

So far, the special counsel’s office has charged 32 people, and six have pleaded guilty. Though Mueller has shrouded his probe in secrecy, he is pushing to wrap up a substantial portion of his investigative work soon and is referring cases to U.S. attorney’s offices that can handle prosecutions once the special counsel probe is disbanded, according to those familiar with Mueller’s work who spoke on the condition of anonymity to discuss sensitive legal deliberations.

A grand jury still seems to be actively investigating Trump associate Roger Stone, and the special counsel’s office is still negotiating with the president’s legal team over the possibility of interviewing Trump himself. Stone said in a statement after the plea: “I am uncertain of the details of Paul’s plea deal but certain it has no bearing on me since neither Paul Manafort or anyone else can testify truthfully that I am involved in Russian collusion, WikiLeaks collaboration or any other illegal act pertaining to the 2016 election.”

Trump and the special counsel’s office could come to a resolution at any moment on Trump answering questions, those involved in the discussions say, but remain at the same basic standstill. Trump’s lawyers don’t want their client to sit down for a face-to-face interview out of fear he would be accused of perjury.

In early August, Mueller offered to reduce the scope of questions he would pose, but Trump’s team ultimately rejected the offer, saying it considered questioning the president about possible obstruction of justice to be legally inappropriate. Just before Labor Day, Mueller notified Trump’s lawyers that he would accept written answers to some questions about the campaign and would delay making a decision for now about seeking answers from the president about his time in the White House. Mueller is interested in that later period as part of his probe of whether Trump tried to obstruct the Russia investigation.

While Manafort had previously seemed to be posturing for a pardon — the president praised him on Twitter as a “brave man” after he fought prosecutors at the Virginia trial — it was not immediately clear whether Manafort would be able to maintain that effort after his plea.

Earlier this summer, Trump had sought his lawyer’s advice on pardoning his former aides, including Manafort. But Giuliani said he counseled Trump that he shouldn’t consider such a pardon until after Mueller’s investigation was completed, and the president understood. “He agreed with us,” Giuliani told The Washington Post last month.

Giuliani declined Friday to say whether Trump is leaning toward pardoning Manafort.

“It’s not something to be considered during a pending investigation. The president shares that view,” he said. “That’s our advice to him, and there is no reason to believe he’s changed his mind on it.”

Manuel Roig-Franzia contributed to this report.