– 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 Mueller'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 Donald 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.

On Friday, though, the special counsel finally nabbed his white whale. Manafort, whose role in the Trump campaign and ties to a Russian-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 Rudy Giuliani said Friday that it would 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. 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 Washington that was scheduled to begin in the 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.