Once Flynn began cooperating with prosecutors, he appeared to be in a position to answer some of the biggest lingering questions in the Russia probe: Did Trump direct Flynn to dangle the easing of sanctions—imposed by President Barack Obama in December 2016—in front of Kislyak during the transition period? And did Trump know that Flynn had misled the FBI when he denied ever discussing sanctions with Kislyak? (If Trump knew the extent to which Flynn was in the FBI’s crosshairs when he asked FBI Director James Comey, whom he later fired, to consider “letting Flynn go,” that could dramatically bolster the obstruction case federal prosecutors are thought to be weighing.) Furthermore, why did the White House wait nearly three weeks to fire a high-level adviser who was, according to former Deputy Attorney General Sally Yates, vulnerable to being blackmailed by the Russians?

As such, the news of Flynn’s extensive cooperation with Mueller could unsettle Trump, who hired Flynn as a high-level surrogate and adviser during the election and kept him on through the transition and into the White House. According to Mueller’s filing on Tuesday, Flynn “provided firsthand information about the content and context of interactions between the transition team and Russian government officials.” Flynn was in the national-security-adviser job for a little more than three weeks before reports surfaced that he had discussed the issue of sanctions with Kislyak during the transition period, despite repeated denials—to Vice President Mike Pence and the FBI—that the topic had ever come up. Trump has also said he did not know about Flynn’s lies to the FBI when he fired him in February 2017—a point that Flynn was in a position to either confirm or deny to the special counsel.

Flynn also may have had knowledge about a “peace plan” that involved lifting sanctions on Russia in return for Moscow withdrawing its support for pro-Russia separatists in eastern Ukraine, which was allegedly hand-delivered to him by the president’s longtime lawyer Michael Cohen.

Flynn’s lawyer, Robert Kelner, alluded to his client’s value in a statement last year as he tried to negotiate immunity deals with the FBI and Congress. “General Flynn certainly has a story to tell, and he very much wants to tell it, should the circumstances permit,” Kelner said. Indeed, from the beginning there were signs pointing to Flynn’s high value as a cooperator. Flynn failed to register as a foreign agent in 2016 when he was representing Turkish government interests, and failed to disclose payments from Russia’s state-owned news agency, Russia Today, when he was renewing his security clearance in January 2016. He also failed to disclose his efforts in 2015 and 2016 to develop nuclear reactors across the Middle East—a joint project with Russia and Saudi Arabia that would have benefited by lifting the sanctions on Russia. Still, Flynn was ultimately charged only with one count of lying to the FBI.