Just 17 months after leading chants of “lock her up” at the Republican National Convention, protesting FBI director James Comey’s handling of the Hillary Clinton email investigation, retired Lt. Gen. Michael Flynn himself faced the inside of a Washington courtroom Friday morning.

Flynn pleaded guilty to a single charge of making false statements, stemming from a nearly year-long probe into his dealings during the presidential transition with Russian ambassador Sergey Kislyak, thus becoming the fourth Trump campaign official to face charges in recent weeks. Based on how the Mueller investigation has progressed so far, he seems unlikely to be the last.

That same investigation into Flynn ultimately culminated earlier this year to Comey’s firing by Donald Trump, just weeks after Trump in a private conversation asked Comey to ease off Flynn, telling the FBI director, “I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy. I hope you can let this go.”

Friday’s guilty plea—and Flynn’s run from a black SUV through a gauntlet of TV cameras in front of the DC court—mark a stunning fall for a man that many once considered one of the great US intelligence officers of his generation.

“My guilty plea and agreement to cooperate with the Special Counsel’s Office reflect a decision I made in the best interests of my family and of our country,” Flynn said Friday in a statement. “I accept full responsibility for my actions.”

Mueller’s acceptance of Flynn’s single charge, in fact, indicates that the former national security adviser has provided—or plans to provide—significant cooperation to Mueller’s team.

Flynn’s plea makes him one of the highest-ranking government officials to face charges since Iran-Contra in the 1980s, when Defense secretary Caspar Weinberger and Flynn’s predecessor as national security adviser, Robert “Bud” McFarlane, both faced indictment. However, the actions by Flynn—and what they potentially point to—make Watergate a better political analog, as Mueller’s fast-moving investigation entered the White House itself for the first time.

The special counsel’s office reported Friday that Flynn had misled FBI investigators about his contacts with the Russian government and—even more ominously for the president—that Flynn said he had been directed to make the contacts by Trump’s team at Mar-a-Lago. One of Mueller’s prosecutors, Brandon L. Van Grack, a veteran national security investigator who specializes in espionage cases, said in court that “a very senior member of the transition team directed” Flynn to contact the Russian ambassador at one point.

The move by Mueller, to accept a guilty plea on a run-of-the-mill “1001 violation,” named after 18 USC § 1001, the criminal statute for making false statements, belies a year of troubling news reports about Flynn’s dealings both with Russia and, particularly, Turkey, a country he retroactively admitted had been paying him even as he worked on Trump’s campaign last year—a violation of the Foreign Agent Registration Act. More recently, The Wall Street Journal has reported that Flynn and his son were allegedly involved in a conspiracy potentially aimed at the kidnapping of a Turkish dissident from US soil.

Mueller’s acceptance of Flynn’s single charge, in fact, indicates that the former national security adviser has provided—or plans to provide—significant cooperation to Mueller’s team. That in and of itself is notable: Since defendants rarely receive much credit for “cooperating down” on lesser targets or their peers, Flynn’s role on the campaign and at the White House means that there are only a narrow number of potential targets for which Mueller might be seeking Flynn’s help—almost all of whom are members of Trump’s own family.

There’s plenty of reason to believe, in fact, that Bob Mueller and his team of prosecutors in the special counsel’s office are only getting started. Based on news reports, it appears that there are entire avenues of Mueller’s investigation that still haven’t seen the light of day—including a wide-ranging search warrant to Facebook about the influence Russian advertising on the platform during the election, as well active investigations centering around Wikileaks, the Trump campaign’s data company Cambridge Analytica, and campaign advisers like Carter Page, among others.