(CNN) Silent for so long, Robert Mueller is beginning to show the public the building blocks of a case that could imperil Donald Trump's presidency.

In the process, the special counsel is beginning to expose the lies and obfuscations that people around Trump, and the President himself, erected to try to hide multiple, unexplained ties to Russians in, and before, 2016.

He is offering implicit explanations along the way for the President's oddly solicitous relationship with Russian President Vladimir Putin.

And Mueller is now crossing a red line Trump once warned could prompt his firing -- by probing his business empire.

By securing a cooperation agreement with Trump's former lawyer, Michael Cohen , Mueller Thursday scored a motivated witness who has intimate knowledge of Trump's past business and personal life.

And by sponsoring a court document detailing Cohen's confession and by having his team spend 70 hours acquiring additional testimony, Mueller is signaling his new star witness may have more to tell and there may be more grave revelations to come.

In fact, Thursday may have been the most significant day yet in the Mueller probe that has cast a long shadow over Trump's presidency.

The cooperation agreement could offer documents, other evidence and testimony that could take Mueller deep into Trump's family and personal circle.

Cohen's admission that he had lied to Congress about a Trump effort to seal a deal to build a Trump Tower in Moscow, deep into the presidential campaign in 2016, raised a flurry of troubling questions that Mueller is yet to answer.

In effect, it told a story of a presidential candidate who was enmeshed in a commercial relationship with a nation that Mueller accused in previous indictments of waging "information warfare" against the United States to disrupt the election and help put Trump into the White House.

The dramatic development came in a week when it became clear that Mueller is aggressively pursuing another avenue in the investigation -- the possibility that some other Trump associates may have communicated with WikiLeaks, the website used to display Clinton campaign emails stolen by Russia and used by Trump to attack the character of the Democratic nominee.

"The two big picture questions have been -- was there any person who was acting as a link between the campaign and Russia or Wikileaks, and what incentive did Trump have to cooperate with the Russians and why is he so beholden to the Russians?" said Jens David Ohlin, vice dean of Cornell Law School.

"I think we now have potential answers to both those questions."

What Mueller has not yet proven

Mueller has yet to prove that Trump's business activities colored his approach to Russia. He has not directly contradicted Trump's fervent denials of collusion between his campaign and Moscow.

Furthermore, it is not known whether Mueller has documentary evidence to implicate the President in any wrongdoing, or is simply relying on Cohen's testimony. Many legal experts doubt he would wager on a witness like Cohen, who has admitted lying, without supporting evidence.

But there is no clear indication that Trump broke the law. And, despite Thursday's developments, there is no sign that a political situation in Washington that still makes impeachment an unlikely prospect has changed.

Trump, just after landing in Buenos Aires for the G20, showed that the investigation was still on his mind despite the upcoming conference.

"This is an illegal Hoax that should be ended immediately," the President tweeted late Thursday.

But Thursday's developments must also be seen in the context of Mueller's work so far.

In a series of indictments, he has built a picture of a sophisticated Russian hacking operation, an attempt to infect America's political dialogue with lies and distraction on social media and indicted and convicted a number of Trump associates and advisers over lying to his investigation.

He may now be trying to establish that Trump and those around him were well aware of Russia's activity despite their vehement claims there was no collusion.

Some observers believe that the rich detail in Mueller's legal filings is one way of painting a picture of Russian interference and the behavior of Trump world in case the President finds some way to block his eventual final report.

There are immediate and longer term political and legal consequences from Cohen throwing himself firmly in Mueller's camp.

His claims, made under oath and with the certainty of a long jail sentence if he is being untruthful, directly contradict Trump's assurances that he has had no deals or business relationships with Russia.

If that is the case, Trump has brazenly lied to the American people.

In another damaging blow, Cohen said he lied for political reasons.

"I made these statements to be consistent with Individual-1's political messaging and to be loyal to Individual-1," Cohen said in a document filed with the court, referring to the President.

His statement raises another question not answered in the document.

Did the President know that Cohen was lying to Congress, or did he coerce him to do so? If he did, such a move would surely rise to the level of the kind of abuse of power that would be part of any articles of impeachment.

The President slammed Cohen as "weak" on Thursday and said he was lying to Mueller to spare himself a long jail sentence after admitting to tax and financial fraud. He made the remarks to reporters before heading off to the G20 summit in Argentina, in an unsolicited statement that appeared to show his unease.

Trump's lawyer Rudy Giuliani also blasted Cohen.

And he tried to scotch a question that buzzed around Washington immediately after the Cohen news: Did Trump tell a different story about his hopes of building a Trump tower in Moscow in written answers to the special counsel than what Cohen has testified, a version of events Mueller clearly believes?

Giuliani said that there was "no contradiction" between Trump's responses to Mueller and Cohen. And he suggested that by waiting to move on Cohen until the President turned over his answers to his questions in recent days, he might have been setting a trap.

"Their sneakiness didn't work if that's what they were trying to do," Giuliani said.

Trump's defense may have backfired

Yet Trump's own self-defense raised another question that could weaken his position and cause trouble for his legal team.

He said that he was justified in seeking business opportunities in Russia while running for President because he might not have reached the White House and should not therefore be penalized by losing a chance to make money.

"There would be nothing wrong if I did do it. I was running my business while I was campaigning," Trump told reporters.

"There was a good chance that I wouldn't have won, in which case I would have gone back into the business, and why should I lose lots of opportunities?"

Trump's comment left the impression he was using his platform as a presidential candidate as a vehicle to enrich himself rather than to serve the American people.

More troublingly, the possibility that he could be seeking favorable treatment from Russia could offer a motivation for a change made to water down a hostile stance towards Russia in the platform at the Republican National Convention.

And the fact that he lied about not having business links with Russia after pursuing the deal offered Moscow leverage over him when he became President, opening the possibility of a serious national security threat if he was viewed as compromised.

Trump might have also undermined a possible avenue of defense, that Cohen was the primary actor in the search for a Trump Tower Moscow deal, when he said: "I decided not to do the project, so I didn't do it. "

Some Trump supporters warned against irresponsibly jumping to conclusions and questioned Cohen's credibility.

"The sky is falling all of a sudden because there is another plea deal. Who knows how this is going to play out in terms of the credibility of Michael Cohen," said Jim Schultz, a former Trump White House lawyer on "Cuomo Prime Time."

The White House was braced for some kind of Cohen bombshell ever since he appeared in Washington to meet Mueller's prosecutors earlier this month.

What must most worry them now, and other people close to Trump who could be implicated by the Russia organization, is what he has said in consultations that have now stretched over three full days.