WASHINGTON (AP) — Former Trump national security adviser Michael Flynn pleads guilty to lying to FBI about talk with Russian ambassador.

It’s the first plea by any of the four former advisers to President Donald Trump charged so far in the wide-ranging investigation led by special counsel Robert Mueller.

Flynn has been under investigation for a wide range of allegations, including his lobbying work on behalf of Turkey, but the fact that he was charged only with a single count of making false statements suggests he is cooperating with Mueller’s investigation in exchange for leniency.

Michael Flynn was President Donald Trump’s favorite general, rapidly vaulted to prominence by his fiery speech at the 2016 Republican Convention and Trump’s decision to reward him with a plum job as his top national security aide.

Flynn’s plunge was even faster. He was fired by Trump after just a month in the White House and left to contend with a criminal probe that led to his decision to plead guilty Friday to a single count of making false statements to the FBI.

Flynn’s decision to cooperate with special counsel Robert Mueller’s investigation signals the role he’s likely to play in expanding efforts to unravel the possible involvement of senior Trump officials and perhaps the president himself in Russia’s meddling in the 2016 presidential campaign.