Michael Flynn, who was fired as President Donald Trump’s national security adviser after revelations that he lied about his communications with Russia’s ambassador, pleaded guilty Friday to charges that he lied to the FBI about those conversations.

"Yes, sir," the retired three-star general and former military intelligence chief said when asked in court if he wanted to plead guilty.

Flynn, 58, is the most senior former U.S, official to be charged in Special Counsel Robert Mueller's ongoing probe into possible collusion by Trump aides with Russia's meddling in the 2016 US election.

Mueller has already secured indictments against former Trump campaign chairman Paul Manafort and his deputy, as well as a guilty plea from a junior foreign policy adviser.

The single charge is outlined in a criminal information unsealed Friday in a Washington, D.C., court.

Flynn was forced to resign less than a month into Trump’s term. The White House was warned by the Justice Department that Flynn could be subjected to blackmail because his dealings with Russians hadn’t been disclosed.

After he left the administration, Flynn filed an updated foreign registration form showing that he hadn’t disclosed multiple contacts and payments from foreign entities while serving as an campaign adviser to Trump starting in February 2016.

At the time, Flynn, a retired Army general, ran a consulting business called Flynn Intel Group. In one case, Flynn’s company received $530,000 from Inovo BV, a Dutch company working on behalf of Turkey’s government, to lobby the U.S. for extradition of a dissident cleric who has opposed President Recep Erdogan of Turkey.

He’s also disclosed payments from RT, described in an unclassified U.S. intelligence report as “the Kremlin’s principal international propaganda outlet,” and Kaspersky Government Security, a cybersecurity business that U.S. authorities say works closely with Russia’s main intelligence agency, the FSB.

Flynn initially drew scrutiny for a secret meeting that he and Jared Kushner, Trump’s son-in-law and senior adviser, had with Ambassador Sergey Kislyak of Russia last December in Trump Tower. Agents for the Federal Bureau of Investigation later asked Flynn in January whether he had talked with Kislyak about sanctions imposed by President Barack Obama in retaliation for Russia’s election meddling.

Flynn, who as a private citizen during those conversations was barred from negotiating with foreign powers, told the agents that sanctions hadn’t come up, the Washington Post has reported. The Justice Department informed the White House that Flynn’s denial contradicted the contents of phone calls intercepted by intelligence agencies, potentially exposing him to blackmail by Moscow, the newspaper reported, citing unnamed current and former U.S. officials.

Flynn resigned on Feb. 13 after only 24 days on the job. In his resignation letter he apologized to the president and vice president for giving them “incomplete information” about his interactions with the Russian ambassador.

Flynn has since drawn criticism for failing to disclose his consulting work for foreign governments while he was the top defense adviser to Trump’s presidential campaign and serving in the transition and cabinet. Top House Democrats say that Flynn failed to disclose a 2015 Middle East business trip tied to a plan to build nuclear plants in the region using money from Saudi and Russian investors. The Democrats called the omission a crime.

Flynn is a retired Army lieutenant general who rose to the top of the military’s intelligence apparatus during a career notable for both his battlefield successes and his breeches of the Pentagon’s chain-of-command.

After he was fired by Obama in 2014 for bucking his military superiors, Flynn started a private lobbying and consulting practice that did business in foreign countries including Russia and Turkey. Flynn didn’t disclose those contacts and payments, as required, when applying for his security clearance to work in the Trump White House, even though several of his clients worked on behalf of the Russian and Turkish governments.

The FBI’s investigation of Flynn gave rise to the appointment of a special counsel to oversee the wider Russia probe. Even after Flynn’s firing, Trump defended him publicly, calling him a “very good person” who had done nothing wrong. In private, Trump asked his FBI director, James Comey, to end the examination of Flynn, Comey has testified. After Comey refused, he was fired, he said. Soon after, Mueller was appointed as special counsel.

The charge against Flynn casts a renewed spotlight on Attorney General Jeff Sessions. Sessions recused himself from any investigation into Russia’s attempts to influence the 2016 election after Democratic lawmakers accused him of lying to Congress about his own conversations with Russian officials.

Flynn’s 33-year military career stretched from the 1983 Granada invasion, where he was a platoon leader, to stints as director of intelligence for the U.S. Central Command, the Joint Chiefs of Staff and the International Security Assistance Force. He retired in 2014 after Obama fired him as director of the Defense Intelligence Agency.

This report contains material from Bloomberg News and AFP.