Former National Security Advisor Michael T. Flynn has been charged by Special Counsel Robert Mueller in federal court in Washington DC with one count of making a false statement to the FBI about his Russian contacts.

The court documents suggest that the charge is part of a plea agreement between Flynn and Mueller, suggesting Mueller has secured Flynn’s cooperation. The cooperation of Flynn, a former top Trump campaign, transition and White House official, marks a grave turn to the Russia investigation for the Trump White House.

The information charges Flynn with one count of making a false statement.

Flynn was fired from the White House in February for obscuring those contacts with Russians during the campaign, and has been under scrutiny for other shady dealings. He is the fourth Trump associate to face charges in Mueller’s probe.

According to the court documents Flynn’s false statements included the claim that he did not talk to Russian Ambassador Sergey Kislyak about sanctions the Obama administration was imposing on Russia in late December. He also made a false statement about talking to Kislyak about a United Nations vote a week earlier.

The case was assigned to U.S. District Judge Rudolph Contreras on Thursday, but the filing itself only became public Friday morning. Flynn is scheduled to appear in front of Contreras at 10:30 Friday morning.

In the months leading up to this week’s charges, Flynn came under scrutiny not just for his Russian contacts, but for business dealings for a project on behalf of the Turkish government that he did not initially disclose. The charges filed Friday did not address those allegations.

Flynn has long been a closely watched figure in Mueller’s probe, and weeks ago it was reported that enough evidence had been assembled to bring charges against him. As early as December 2016, the FBI was investigating Flynn, according to reports, and in January 2017, Sally Yates, a top Justice Department official at the time, raised concerns with the new Trump White House that Flynn was lying about his contacts with Russians during the campaign.

As the Mueller probe picked up, Flynn’s lobbying associates and other business associates were subpoenaed and contacted by the investigation. Flynn’s son, Mike Flynn Jr., who served as an aide to Flynn in his business ventures, has also reportedly been under scrutiny. It does not appear Mueller has brought any charges against Flynn Jr. at this point.

Mueller’s interest in Flynn has also said to include his contacts with Peter Smith, an elderly GOP operative who hunted for Clinton’s emails during the campaign and since committed suicide.

At the heart of the feds’ concerns about Flynn were his thicket of foreign business entanglements, their effect on his policy decisions while in government, and his alleged efforts to conceal them.

As Yates revealed in congressional testimony, the Justice Department believed Flynn was “compromised with regards to the Russians” for lying to FBI agents about his regular contacts with Russian officials. Those included calls with then-Kremlin Ambassador Sergey Kislyak about easing U.S. economic sanctions against Russia and sitting in on a meeting with Kislyak and Trump’s son-in-law Jared Kushner during the transition to discuss establishing a secret communications backchannel.

The other primary foreign source of Flynn’s legal troubles was Turkey. Flynn and his firm, Flynn Intel Group, made a rather astonishing series of moves to take down exiled Muslim cleric Fethullah Gulen, who Turkish President Recep Tayyip Erdogan believes orchestrated a failed 2016 coup attempt against him. Flynn accepted $530,000 from a Turkish businessman to create and disseminate negative PR materials about Gulen during the final months of the campaign and into the transition—a move that forced Flynn to retroactively register as a foreign agent. He also allegedly collaborated on secret discussions with senior Turkish officials to plot how to spirit Gulen back to Turkey from the U.S., possibly in exchange for huge sums of cash.

Other possible quid pro quo questions circled around Flynn’s brazen promotion, while in office, of a multi-billion-dollar plan pushed by his former business associates to build and operate dozens of nuclear plants in Saudi Arabia and other Middle Eastern countries.

Top White House officials, most notably Trump, stayed loyal to Flynn even as these damning details emerged in the press. The President personally intervened on Flynn’s behalf the day after he was fired, urging then-FBI director James Comey in a private Oval Office meeting to “let [Flynn] go.” Comey declined to do so, or to help lift what Trump called the “cloud” caused by the Russia investigation, and was abruptly fired as a result in May, prompting Mueller’s appointment.

What Flynn brings to Mueller’s probe is also a firsthand view of what was happening in Trump’s inner circle through the bulk of the 2016 campaign, as well as during the transition and into the early days of the administration.

Flynn, a decorated retired general who served as the Director of the Defense Intelligence Agency before being fired by President Obama, began advising the Trump campaign in February 2016. He soon became a key player in Trump’s inner circle and a mainstay on the trail, where he revved up crowds of Trump supporters with anti-Hillary Clinton chants of “Lock Her Up.” Flynn joined Trump during his first classified briefing, and Trump designated Flynn as national security advisor in one of his first major staff announcements after Election Day. He lasted in the position for two weeks, until he was pushed out for allegedly lying to Vice President Mike Pence about his Russian contacts.

Mueller’s move comes after he secured indictments for three other associates of Trump’s 2016 campaign. Campaign foreign policy advisor George Papadopoulos pleaded guilty in October to lying to FBI officers about his Russian contacts in 2016. Former Trump campaign chairman Paul Manafort and his longtime deputy Rick Gates both face charges of money laundering, tax evasion and failure to disclose foreign lobby. They have pleaded not guilty.

Read Flynn filing below: