(CNN) Former national security adviser Michael Flynn's decision to plead guilty to a single count of lying to the FBI over conversations he had with the Russian ambassador during the 2016 election is an absolutely massive moment in the ongoing special counsel probe being led by Robert Mueller -- and has potentially huge ramifications for President Donald Trump.

Flynn's decision to cop a plea and cooperate with investigators -- coupled with the fact that the lone charge against him deals with lying to the FBI as opposed to his dealings with Turkey -- suggests that Mueller believes there could be someone higher in the chain of command than Flynn about whom the former national security adviser can provide critical information.

Which is saying something.

Flynn was, from the start of Trump's campaign, a member of the inner circle -- a loyal defender and trusted confidante of Trump. When Trump won, Flynn was given carte blanche in terms of what role he wanted to play in the White House.

Given that, the guilty plea by Flynn is different -- and more serious -- than the charges against one-time Trump campaign chairman Paul Manafort . Manafort was a part of Trump's inner orbit for a handful of months during the spring and summer of 2016. But, he was long gone by the time Trump won the White House. And, the charges against Manafort have to do with money laundering and the Ukraine, not Russia. Manafort has pleaded not guilty.

It's also impossible for Trump to dismiss the role Flynn played in his campaign and White House as he did with George Papadopoulos, who pleaded guilty to lying to the FBI about interactions with foreign officials close to the Russian government.

The Flynn guilty plea comes from someone who, until the day he was reluctantly fired by Trump as national security adviser, sat at the absolute epicenter of Trumpworld. And, unlike Manafort, Flynn's charge deals directly with his interactions with the Russian ambassador -- and goes to the very core of Mueller's investigation into Russia's interference in the 2016 election and any possible collusion between Russia and the Trump campaign.

In short: The calls are now coming from inside Donald Trump's house.

The big question going forward is who is the real target (or targets) of Mueller's investigation -- and how Mueller believes what Flynn knows helps make that case.

Is it Trump son-in-law Jared Kushner? We know the role Kushner played in the firing of FBI Director James Comey has been a subject of questions from Mueller's investigators to witnesses before them. We know from reports in The New York Times and Wall Street Journal that Mueller's team is looking into Jared Kushner's activities surrounding the December 2016 UN Security Council vote that condemned Israeli settlements. The charges against Flynn seem to reference that vote.

Is it Donald Trump Jr? We know the eldest son of the President -- as well as Kushner -- participated in a July 2016 meeting at Trump Tower with several Russians -- a meeting spurred by the promise that dirt would be provided on Hillary Clinton.

We don't know the answer to that "who is it" question yet. What we do know, however, is that Flynn's plea deal with Mueller brings the special counsel investigation ever closer to the President of the United States. (Worth noting: As of the spring -- prior to the firing of Comey by Trump and the appointment of Mueller as special counsel -- Trump was not under FBI investigation. We know that because Trump says Comey, on several occasions, told him he wasn't.)

Whether or not this probe ultimately reaches to Trump, it's now totally impossible for the President and his allies to dismiss the Mueller investigation as a "hoax," a distraction or the work of Democrats bitter about losing the election.

The following things are facts:

Mueller was appointed as special counsel by Trump Justice Department deputy Attorney General Rod Rosenstein. Flynn was an extremely close and influential adviser to Trump as a candidate and as president. Flynn pleaded guilty to lying to the FBI about his interactions with Russian ambassador Sergey Kislyak.

There's no "fake news" media in either of those three sentences. Or Democrats. Or hoaxes. Or witch hunts.

That doesn't mean that Trump, when he tweets his thoughts on Flynn's guilty plea -- and he will tweet about Flynn's guilty plea -- won't continue to dismiss the entire investigation as a farce. He almost certainly will.

But, that doesn't make it true. And, with the Flynn guilty plea, there is absolutely no thinking person who could possibly believe that Trump's presidency is not in some level of peril now.

Peril does not mean Trump is a goner. But, it does mean that Mueller's Russia investigation almost certainly will get worse for Trump before -- and if -- it gets better.

The Flynn charge is unmistakeable proof that this whole thing is inside Trump's White House. And Flynn's guilty plea is unmistakeable proof that it's not going to be leaving the White House any time soon.

And that is -- no matter how many tweets he sends -- absolutely awful news for Donald Trump.