WASHINGTON ― Former national security adviser Michael Flynn, the man President Donald Trump trusted with the nation’s closest-held secrets, is now a felon, admitting in federal court Friday that he lied to FBI agents just days after his former boss was sworn in as president. But it’s actually the relative non-severity of the charge against Flynn that should have Trump and his top aides worried.

Flynn, who served as Trump’s national security adviser for 24 days before he was fired for reportedly lying to Vice President Mike Pence about the content of his discussions with the Russian ambassador weeks before Trump’s inauguration, had significant legal exposure. As did his son Michael Flynn Jr. So the fact that he struck a plea deal with just a single count that could potentially mean no prison time indicates he’s been able to leverage significant information on an even bigger target. In short, special counsel Robert Mueller’s team isn’t done yet.

“Usually in government investigations they don’t cooperate down, they cooperate up,” Mark Lee, a former federal prosecutor, told HuffPost. “Gen. Flynn is probably a medium-sized fish. They’re looking for a bigger fish than him.”

Jonathan Ernst / Reuters Former national security adviser Michael Flynn leaves U.S. District Court in Washington on Friday morning. It looks like a bigger shoe could soon drop.

Trump has dismissed Mueller’s probe as a “witch hunt” and a “hoax,” and the White House is already trying to distance the administration from Flynn. But the investigation has already resulted in two guilty pleas ― from Flynn and former Trump campaign adviser George Papadopoulos ― as well as hefty criminal charges against Trump’s former campaign manager Paul Manafort and his deputy Rick Gates. Now, armed with the cooperation of Flynn and Papadopoulos and with significant leverage against Manafort and Gates, Mueller’s team appears poised for another takedown.

“Flynn would not be permitted to enter a plea bargain unless he had substantial credible evidence of criminal activity involving high-level members of the Trump campaign, including potentially the president himself,” Notre Dame Law School professor Jimmy Gurulé said. “Clearly this is not good news for President Trump.”

Two individuals specifically referenced in court documents in connection with Flynn’s guilty plea Friday should be particularly nervous.

First, there’s a “senior official” on the Trump transition team who was at Trump’s Mar-a-Lago resort on Dec. 29. According to his statement of offense, Flynn made a call that day to “a senior official” who was “with other senior members” of the transition team at Mar-a-Lago. This was the day after the Obama administration had announced sanctions against Russia in response to Russia’s interference in the 2016 election and Russian ambassador Sergey Kislyak had contacted Flynn for a chat.

In that Dec. 29 call, Flynn and other members of the Trump transition team, according to the statement of offense, “discussed the U.S. Sanctions, including the potential impact of those sanctions on the incoming administration’s foreign policy goals.” Then, “immediately” after talking to the official, Flynn called the Russian ambassador and asked that Russia not escalate the situation. Fox News, CNN and ABC reported Friday that K.T. McFarland, a former top administration official Trump has nominated to become an ambassador, was the “senior official” in question.

Mark Wilson via Getty Images

Second, there’s the “very senior member of the Presidential Transition Team” who, on Dec. 22, directed Flynn to call other countries, including Russia, to learn where governments stood on a U.N. Security Council resolution on Israeli settlements. Multiple outlets have reported that this is Jared Kushner, Trump’s son-in-law and adviser. Flynn, according to his statement of offense, requested that Russia either vote against or delay the resolution. Flynn admitted he lied when he told FBI agents that he asked only for their position on the vote and didn’t request they take any particular action.

Flynn’s admissions that he acted on behalf of the Trump transition team to interfere with U.S. foreign policy raises the potential of charges under the Logan Act, a never-used 1799 statute than bans citizens from interfering in foreign relations, as well as potential other criminal statutes.

As part of his plea deal, Flynn has agreed to “cooperate fully, truthfully, completely, and forthrightly” with Mueller’s office or any other federal, state and local law enforcement authorities Mueller’s office identifies where Flynn’s cooperation would be relevant. Flynn also must “promptly” turn over “any and all evidence of crimes” of which he’s aware.

Ryan Reilly is HuffPost’s senior justice reporter, covering criminal justice, federal law enforcement and legal affairs. Have a tip? Reach him at ryan.reilly@huffpost.com or on Signal at (202) 527-9261.