During his much-anticipated public testimony before Congress on Wednesday, President Donald Trump's former longtime attorney Michael Cohen will provide evidence of his old boss's alleged criminal conduct since becoming president, a knowledgeable source told NBC News.

And, according to the source, there is much more that congressional investigators and viewers can expect Cohen to provide.

He will detail his allegations of the president's lies, racism and cheating as a private businessman while Cohen spent a decade working for him, the source said.

Cohen will also give lawmakers information about Trump's financial statements and might actually provide the statements, the source said. This information would require the president's long-held secret tax returns in order to verify, providing an avenue for the House Committee on Oversight and Government Reform to request those returns. The source noted the allegation that Trump deflated the value of his properties in some cases to reduce his property taxes.

Cohen's three-day appearance began Tuesday with closed-door testimony before the Senate Intelligence Committee. Only the Wednesday hearing before the House Oversight Committee is scheduled to be made public. Cohen will also testify before the House Intelligence Committee on Thursday.

Michael Cohen arrives at the Hart Senate Office Building on Tuesday. Chip Somodevilla / Getty Images

According to a source close to Cohen, he started his appearance before the Senate committee on Tuesday by saying he wanted to apologize for his past testimony and to correct the mistakes made and correct the record.

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Cohen will in his testimony this week address his motives for lying on behalf of Trump and why he lied to Congress about the Trump Tower Moscow negotiations — a charge to which he pleaded guilty late last year, a knowledgeable source told NBC. He will additionally discuss whether anyone told him to lie. After Cohen pleaded guilty, Trump defended his efforts to build a Trump-branded tower in Russia while running for president of the United States as "very legal" and "very cool."

The hush-money payments will be at the forefront of Cohen's public testimony, too, this source said. Cohen is set to provide intimate details of any involvement Trump had in the scheme to keep porn star Stormy Daniels quiet about the affair she said she had with Trump in 2006 by paying her $130,000 just before the 2016 election. Cohen will discuss the reimbursement he received for the payment as well.

White House press secretary Sarah Sanders said in a statement that it was "pathetic" Cohen was being brought in to testify.

“Disgraced felon Michael Cohen is going to prison for lying to Congress and making other false statements," Sanders said. "Sadly, he will go before Congress this week and we can expect more of the same. It’s laughable that anyone would take a convicted liar like Cohen at his word, and pathetic to see him given yet another opportunity to spread his lies.”

As he arrived for Cohen's closed-door testimony Tuesday morning, Senate Intelligence Committee Chairman Richard Burr, R-N.C., said the most important thing the panel could learn is the "truth," but added that Cohen's track record was "questionable" on that front.

Committee Vice Chair Mark Warner, D-Va., said toward the end of Cohen's testimony that "the only comment I am going to make is two years ago, when this investigation started, I said it may be the most important thing I am involved in my public life in the Senate, and nothing I have heard today dissuades me from that view."

Sen. Ron Wyden, D-Ore., a member of the panel, said earlier in the day that he planned to focus on “untangling the very complicated financial arrangements between Donald Trump and Russia.”

Sen. Roy Blunt, R-Mo., meanwhile, said in the afternoon that the tone of the hearing was "serious" and questions focused initially on Cohen's previous false statements.

The House Oversight Committee earlier provided the framework for what members want Cohen to address during his open testimony. After the testimony was scheduled last week, Cohen tweeted: "Looking forward to the #American people hearing my story in my voice!"

Rep. Jackie Speier, D-Calif., who sits on both the Oversight and Intelligence Committees, told MSNBC that the transcript of Cohen's private testimony Thursday "eventually" would be made public and said questions would focus on Russia and Trump's plans for a tower in Moscow.

Trump and his legal team have repeatedly called Cohen a liar. They've said he is not a credible witness and is just saying whatever he feels necessary to reduce his prison time. Trump has also publicly targeted other members of Cohen's family, suggesting investigators should look into them. In response to Cohen's allegation that Trump tried to intimidate him, the president told reporters last month that Cohen has "only been threatened by the truth."

In December, Cohen was sentenced to three years in prison for what a Manhattan federal court judge called a "veritable smorgasbord" of criminal conduct, including facilitating secret payments to women who claimed they had affairs with Trump, lying to Congress about the president's business dealings with Russia and failing to report millions of dollars in income.